Building RTL-Aware Web Apps & Websites: Part 2

Pushing the Web forward means making it better for developers and users alike. It means tackling issues that our present Web faces; this is especially true for making responsive RTL design and development easier to achieve.

Before we move on to bleeding edge RTL topics, here’s a video tutorial with a practical example to refresh your mind on how to code RTL properly today:


Picking up where we left off in our last post, it’s time to dive into some advanced topics around Right-To-Left UI implementation. In Part 1 we covered the basics of RTL. In this second post we will cover how-tos for bidirectional (BIDI) user interfaces on the bleeding edge. This is the alpha state of the Web, and we can take a look at what the future of RTL holds in store.

RTL on the bleeding edge

The core of RTL UI design and development on the Web is found in CSS properties and, as with every other aspect of web development, these properties keep improving over time.

Here are the CSS properties you can start using today in alpha versions of several modern browsers. (This information is likely to change as browser updates are released.):

Text alignment: In the past we used to explicitly set the horizontal alignment of text with text-align left or right (e.g. text-align: left). We now have start and end values that can be used instead. For instance, text-align: start will give a different result depending on whether the content is set to RTL or LTR. start indicates left in LTR and right in RTL, and vice versa for end.

Example using left/right:

#content {
  text-align: left:
html[dir="rtl"] #content {
  text-align: right;

Example using start/end:

#content {
  text-align: start;

With start/end, there’s no need for an RTL rule.
Browser support: Chrome 1.0+, Safari 3.1+, Firefox 3.6+, Android, iOS. Not supported by Internet Explorer at this time.

Padding, margin, and border: there are a couple of improvements regarding these properties.
Each of them now can take an additional suffix of -inline-start or -inline-end, for example you can write padding-inline-end or margin-inline-start. These new keywords give the same result as start and end in text alignment: padding-inline-start equates to padding-left in LTR and is padding-right in RTL. margin-inline-end gives the effect of margin-right in LTR and margin-left in RTL.

Example using left/right:

#content {
  padding-left: 12px:
  margin-right: 20px;
html[dir="rtl"] #content {
  padding-left: 0;
  padding-right: 12px;
  margin-left: 20px;
  margin-right: 0px;

Example using start/end:

#content {
  padding-inline-start: 12px:
  margin-inline-end: 20px;

Again, there’s no need for an RTL rule; margin-inline-end will act as margin-right in LTR and act as margin-left for RTL, for example.
Browser support: Firefox 41+ only.

Absolute positioning: left and right are CSS properties that have always been key to absolute positioning, but very soon you’ll be writing much smarter CSS for your users with the new offset-inline-start and offset-inline-end properties.

It is also worth mentioning that top and bottom can be substituted by offset-block-start and offset-block-end.

Example using left/right:

#content {
  position: absolute;
  left: 5rem;
html[dir="rtl"] #content {
  left: auto;
  right: 5rem;

Example using start/end:

#content {
  offset-inline-start: 5rem;

Once again, the start/end concept has saved us a rule in our CSS file.
Browser support: Firefox 41+ only.

Floats: float: left and float: right have been around and used for a long time, but very soon you will be able to use float: inline-start and float: inline-end in Firefox Nightly. inline-start will float your element left in an left-to-right (LTR) layout, and right in a right-to-left (RTL) layout.

Example using start/end:

#content {
  float: left;
html[dir="rtl"] #content {
  float: right;

Using start/end:

#content {
  float: inline-start;

Browser support: Firefox 44 (soon to land in Firefox Nightly).

Firefox OS gives you a great opportunity to try some of the very latest CSS property implementations — there is a reference page on the Mozilla Wiki for all the new BIDI-relevant CSS properties and values.

Web components: Web components are reusable user interface widgets (see MDN for more information). If you’ve worked with web components you’ll know that CSS styles defined inside the shadow DOM are scoped, so by default you can’t get the direction of the overall page from the component’s scoped CSS using html[dir=”rtl”]. However, the shadow DOM introduces a new selector called :host-context(), which can be used to select a shadow host element based on a matching parent element.

Let’s make this clearer with an example — say you have an element inside your shadow DOM tree with a class name called back. In a normal situation where there isn’t a shadow DOM you would just use html[dir=”rtl”] .back {}, however you can’t use this syntax from inside a web component. Scoped and encapsulated, remember? The equivalent selector when .back is inside a web component scope would look like this:

.back:host-context(html[dir="rtl"]) { }

The exact browser support status of this syntax is not clear currently, however you can use this polyfill to plug holes in support.

Want to use a polyfill for shadow CSS but still want to play around with RTL in web components in browsers that already support it such as Firefox (currently hidden behind the dom.webcomponents.enabled flag) and Google Chrome? You can do this using Mutation Observers by observing the document.documentElement.dir property. Code for the observer would look similar to this example:

var dirObserver = new MutationObserver(updateShadowDir);
dirObserver.observe(document.documentElement, {
  attributeFilter: ['dir'],
  attributes: true

Also don’t forget to initialize your function for the first time once the page loads, right after the observer logic:


Your updateShadowDir() function would look something like this:

function updateShadowDir() {
  var internal = myInnerElement.shadowRoot.firstElementChild;
  if (document.documentElement.dir === 'rtl') {
    internal.setAttribute('dir', 'rtl');
  } else {

That’s all you should be doing on the JavaScript side — now, since the first child element of your web component has a changing dir attribute, your CSS will depend on it as a selector instead of the HTML element. Say you are using a span — your selector would look like this:

span[dir="rtl"] rest-of-the-selectors-chain { … }

This might look a little hacky, but fortunately the future is brighter, so let’s take a look at what the W3C has in store for us in the future regarding Right-To-Left.

The future of RTL on the Web

Informing the server of text direction: Currently in text inputs and text areas, when a form is submitted, the direction of the page/form elements is lost. Servers then have to come up with their own logic to estimate the direction of the text that was sent to them.

To solve this problem and be more informative about the text being sent as well as about its direction, W3C has added a new attribute to the HTML5 forms spec called dirname, which browser vendors will be implementing in the future.

As the spec states, including the dirname “on a form control element enables the submission of the directionality of the element, and gives the name of the field that contains this value during form submission. If such an attribute is specified, its value must not be the empty string.”

Translation: Adding this attribute to your <input> or <textarea> makes their direction get passed along with the GET or POST string that is sent to the server. Let’s see a real-world example — first, say you have this form:

<form action="result.php" method="post">
<input name="comment" type="text" dirname="comment.dir"/>
<button type=submit>Send!</button>

After submitting the form, the POST string sent to the server will include a field called comment, and another called comment.dir. If the user types Hello into the text area and submits it the result string will be:

But if they type “?????” it will look like this:


Read the spec here.

Note: In order for Arabic characters to be displayed correctly in URLs in your browser the characters are encoded in UTF-8 and each Arabic letter now uses 4 characters and is separated in the middle with a % sign.

For example, if you have this link:

When you visit it, the link in your browser will be transformed to

Better ways to manage backgrounds and images in RTL: Tired of using transform: scaleX(-1) to mirror your background images? The W3C is introducing a new CSS keyword called rtlflip, which is used like so:

background-image: url(backbutton.png) rtlflip;

This keyword saves you the hassle of manually mirroring your images through the transform: scaleX(-1) property.
It can also be used to style list item markers like this:

list-style-image:url('sprite.png#xywh=10,30,60,20') rtlflip;

The spec page also states that this keyword can be used along with all of the other possible ways that an image can be specified in CSS3 Images, e.g. url, sprite, image-list, linear-gradient, and radial-gradient.

Internationalization APIs

We covered the JavaScript Internationalization APIs before in an introductory post. Let’s recap the important lesser-known parts relevant in the context of working on right-to-left content.

Internationalization API support covers a wide variety of languages and their derivatives. For example it not only supports Arabic as a language, but also Arabic-Egypt, Arabic-Tunisia, and the whole list of other variations.

Using Eastern Arabic numerals instead of Western Arabic: In some Arabic variants, Eastern Arabic numerals (???) are used instead of the Western Arabic (123). The International APIs allow us to specify when to show ??? and when to show 123.

console.log(new Intl.NumberFormat('ar-EG').format(1234567890));

This states that we want to format 1234567890 to Arabic-Egypt numbers.
Output: ?????????????

console.log(new Intl.NumberFormat('ar-TN').format(1234567890));

Here we’re saying we want to output the same number in Arabic Tunisia format; since Tunisia uses Western Arabic, the output is: 1234567890

Showing dates in the Hijri/Islamic calendar instead of the Gregorian: During the Ramadan month, Google made to show content related to the occasion. One of the essential pieces of information shown was the day of the month in the Hijri calendar. Google used a polyfill to get this information. With the Internationalization APIs you can extract the same information with one line of JavaScript instead of a whole JavaScript library.
Here’s an example:

console.log(new Intl.DateTimeFormat("fr-FR-u-ca-islamicc").format(new Date()));
This line states that we want to format the current date to the Islamic Calendar date and show it in fr-FR numerals (Western Arabic). Output: 16/12/1436
console.log(new Intl.DateTimeFormat("ar-SA-u-ca-islamicc").format(new Date()));
This is doing the same thing, but formatted for display in the Arabic-Saudi-Arabia locale which uses Eastern Arabic numerals.
Output: ???/???/????

Final words

In this article, we wrap up our in-depth look at doing RTL on the Web the right way. We hope this has been helpful in encouraging you to get started adding RTL support to your websites/products! If you are a Firefox OS contributor this will be your guide to for adding RTL to Gaia.

As always, let us know in the comments below if you have any questions!

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Building RTL-Aware Web Apps & Websites: Part 1

Making the web more accessible to more people, in more languages, is an ongoing effort and a mission we take very seriously at Mozilla.

This post is the first of a series of articles to explain one of the most neglected and least well-known corners of web development: RTL (right-to-left) development. In a web development context it means making your web content compatible with RTL languages like Arabic, Hebrew, Persian, and Urdu, which are all written from right to left.

This area is often neglected by developers because of the lack of teaching resources. In this series, we hope to provide enough information to allow developers to make content that’s accessible to a broader global audience using the RTL capabilities that most web browsers currently provide.

What actually is RTL?

To make things clear, there is a difference between a language and its script. A script is the text or written form — while the language refers to the spoken form. So technically, right-to-left describing the script in which a language is written, comparable to left-to-right scripts, like English or French, more familiar to many Western readers.

For example, “Marhaban” – which means hello – is a word written in the English script, but has Arabic as its language, while “?????” is both Arabic script and Arabic language.

On the web, as we said earlier, the term Right-To-Left indicates more than just a script for writing. It stands for all facets of developing web apps and websites that do not break or become unreadable when used with Arabic, Urdu or any other RTL script.

Before continuing, let’s just clear up some misconceptions about RTL.

First, RTL is not about translating text to Arabic: It means more than translating your website into Arabic and calling it a day. It’s about making every aspect the UI and layout RTL-friendly. And as I always say – and can’t stress it enough – do RTL right or don’t bother! Doing it halfway will lose your audience and credibility.

Second, RTL is more than just “flip all the things”: I’m not sure if this issue has been fixed yet or not, but setting your locale to Arabic in Gnome will cause the time to be shown as PM 33:12 instead of 12:33 PM.

Well, that is not how it works. I’m a native Arabic speaker but that doesn’t mean I tell time backwards. There are some exceptions and things to pay attention to about numbers, and we will cover them in this series.

Why should I care about RTL?

You might have the impression that RTL is hard, scary, and will add a great deal of work to your project! Really, it’s not that difficult. Once you comprehend the basics, the extra effort required is not that great.

You should really care about adding right-to-left support to your web app or website for many, many reasons. There are over 410 million native RTL speakers around the world as of 2010 (That’s a lot of people! Note: The assessment is based on Wikipedia’s list of all the languages.) It’s also millions of potential users and a huge potential market for your app or website. Most companies now add RTL support to their software (e.g. Windows, iOS, Android) and websites, so native RTL speakers expect your website to meet their expectations regarding RTL support. Without this support visitors to your web application may not become repeat users.

In the case of Firefox OS, we only started shipping devices to the Middle East when we had RTL support fully implemented in the system, and thus our partner Orange was able to sell devices with the OS on it. This obviously opens up a whole new market of users to Firefox OS.

Best practices for RTL

Here are some general rules of thumb to keep in mind when developing RTL-aware websites and web apps:

  • Think from the right: In a region with an RTL script and language, books open from the right-hand side. Thus, most UI elements related to readability are going to be mirrored.
  • Hardware legacy UI is a thing: In MENA (Middle East and North Africa) and other regions where RTL languages are spoken, people use gadgets too. Audio hardware has the same control layout as anywhere else in the world. So buttons like Play, Fast Forward, Next, Previous have always been the same. And because of that, it’s not a good idea to mirror any of those buttons. Please don’t RTL the media player buttons. Another example of hardware legacy is the physical feature phone keyboard; before there were Blackberries and similar keyboards for handhelds, there was the simple physical numpad — until very recently it was widely popular in MENA countries. For this reason, It is strongly advised to *not* mirror the number keys. A good example of this is Firefox OS. As its initial target was developing countries, we made sure not to make the dialer mirrored in RTL.
  • Firefox OS Contacts AppThinking Right-To-Left is not thinking left-handed: Remember not to confuse developing RTL UIs with southpaw-first ones. In RTL regions of the world, right-handed people are a majority, just like in the rest of the world. So anything that is not related to how the user reads the actual content on the page should not be mirrored. An example of this is the Firefox OS A-Z scrollbar (image to the right) in the Contacts App. It is placed on the right because it’s easier to scroll through with the right hand, so such a thing should not go on the other side when your page is in RTL mode.

Down to business: How to RTL content

The first step towards making a web page RTL is adding the code dir="rtl" to the <html> tag:

<html dir="rtl">

dir stands for direction and setting its value to rtl defaults the horizontal starting point of elements to the right instead of the left.

To target an element with CSS only when the page is RTL:

  1. Create a copy of any regular CSS rules that target it.
  2. Add a html[dir="rtl"] attribute selector onto the front of the selector chain.
  3. Next, whenever you find a property or a value that represents horizontal positioning of some kind in the RTL rule, use its opposite. For example, if you find `float: left` you should change it to float: right.

As an example, if the original rule is as follows —

.someClass {
    text-align: left;
    padding: 0 10px 0 0;
    text-decoration: underline;

we would paste a copy of it after the original and update it as follows:

html[dir="rtl"] .someClass {
    text-align: right;
    padding: 0 0 0 10px;

Notice that the text-decoration: underline; declaration has been removed. We don’t need to override it for the RTL version because it doesn’t affect any direction-based layout elements. Thus, the value from the original rule will still be applied.

Here’s an even more detailed example you can hack on directly:

See the Pen dYGKQZ by Ahmed Nefzaoui (@anefzaoui) on CodePen.

Of course, real-life cases won’t be quite as simple as this. There are applications with a huge number of CSS rules and selectors, and there are multiple strategies you could adopt. Here is one recommended approach:

  1. Copy and clean up: First, copy the content of your stylesheet to another file, add html[dir="rtl"] to all the rules, then clear out any properties that do not relate to horizontal direction setting. You should end up with a more lightweight file that deals purely with RTL.
  2. Mirror the styles: Change all of the properties left in the new file to their opposites: e.g. padding-left becomes padding-right, float: right becomes float: left, etc. Note: if you originally had padding-left and you changed it to padding-right, remember that the original padding-left still exists in the original rule. You must add padding-left: unset alongside padding-right, otherwise the browser will compute both properties: the padding-left from the original CSS rule and the new padding-right in the RTL rule. The same is true for any property that has multiple direction variants like the margin-left|right, border-left|right
  3. Paste back: After you’ve completed the previous steps, paste the newly created rules back into the original file, after the originals. I personally like to add a little comment afterwards such as: /* **** RTL Content **** */ for the sake of easier differentiation between the two parts of your style file.

A better way: isolating left/right styles completely

Sometimes you will find yourself facing edge cases that produce code conflict. This involves properties inheriting values from other properties, meaning that sometimes you won’t be sure what to override and what not to. Maybe you’ve added the margin-right to the RTL rule but are not sure what to set as the value for the original margin-left? In all real-world cases it is advisable to adopt another approach, which is better in general but more long-winded. For this approach we isolate direction-based properties completely from the rules for both the LTR and the RTL cases. Here’s an example: say we have .wrapper .boxContainer .fancyBox listed like this:

.wrapper .boxContainer .fancyBox {
    text-align: left;
    padding-left: 10px;
    text-decoration: underline;
    color: #4A8CF7;

Instead of adding another property for RTL with both padding-left and padding-right, you can do this:

.wrapper .boxContainer .fancyBox {
    text-decoration: underline;
    color: #4A8CF7;

html[dir="ltr"] .wrapper .boxContainer .fancyBox {
    text-align: left;
    padding-left: 10px;

html[dir="rtl"] .wrapper .boxContainer .fancyBox {
    text-align: right;
    padding-right: 10px;

This solution consists of 3 parts:

  1. The original rule/selector with only non-direction based properties, because they are shared by the LTR and the RTL layout.
  2. The left to right case — html[dir="ltr"] — with only the direction-based CSS properties included, their values set to match your LTR layout.
  3. The right to left case — html[dir="rtl"] — with the same properties as the LTR case, only with their values set according to what you want in your RTL layout.

Notice that in the second rule we had padding-left only and in the third one we had padding-right only. That’s because each one of them is exclusive to the direction that was given to them in the dir attribute. This is a nice, clean approach that doesn’t require unsetting of properties. This is the exact approach we use when adding RTL support to Firefox OS. (Note: the unset keyword isn’t supported in all browsers. For the best compatibility you may need to explicitly re-declare the desired value for any properties you want to unset.)

How do we get and set the direction of the page using JavaScript?

It’s fairly easy, and requires only one line of code: document.documentElement.dir
You can assign this line to a variable, and have it outputted to the console to see for yourself:  

var direction = document.documentElement.dir; console.log(direction);

Or try this example:

See the Pen WQxWQQ by Ahmed Nefzaoui (@anefzaoui) on CodePen.

To set the direction of the page, just use the following:
document.documentElement.dir = "rtl"

And here’s an example of both getting and setting the direction value for a web page:

See the Pen gaMyPN by Ahmed Nefzaoui (@anefzaoui) on CodePen.

Automatic RTL solutions

Enough talk about manual ways to do RTL. Now let’s take a look at some automated approaches.


Twitter has developed a solution to automate the whole process and make it easier for bigger projects to implement RTL. They have open sourced it and you can find it on Github.

This NPM plugin covers pretty much all the cases you could find yourself facing when working on RTL, including edge cases. Some of its features include:

  • no-flip: To indicate to the mirroring engine that you don’t want a certain property flipped, just add /* @noflip*/ at the beginning of the property. So, for example, if you write /* @noflip*/ float: left, it will stay as float: left after css-flip is run.
  • @replace: When you have background images that differ between LTR and RTL, you can specify a replacement image by including /*@replace: url(my/image/path) before the original property. Let’s say you have background-image: url(arrow-left.png). If you update the line to
    /*@replace: url(arrow-rightish.png) */ background-image: url(arrow-left.png);
    You will end up with background-image: url(arrow-rightish.png); in the RTL layout.

You can use css-flip through the command line by executing
css-flip path/to/file.css > path/to/file.rtl.css or by requiring css-flip in your page. More about that on their github repo. css-flip is being actively used on Twitter properties.


Another option for automatic conversion is a tool from Mohammad Younes called rtlcss — this is also available on github and provides some pretty nifty features. With this one engine you can:

  • Rename rule names (e.g. renaming #boxLeft to #boxRightSide).
  • Completely ignore rules from the mirroring process.
  • Ignore single properties.
  • Append/prepend new values.
  • Insert new property values in between other property values.

Basic usage is via the command line — you can create your RTL CSS equivalent by the running the following in the same directory your original CSS is available in:

rtlcss input.css output.rtl.css

And of course you can just require it in your page. More details are available on github.

This project is very popular and is being used by several projects including WordPress.

Final words

While there is still a lot to cover, this article aims to provide you with enough knowledge to start exploring RTL confidently. In the next article we’ll cover more advanced topics around RTL.

Be sure to ask any questions you may have around RTL in the comments section and we will try to answer them here or in the next post!

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Testing the Firefox browser on mobile websites: Are you game?

Friends and hackers, we have a challenge

Are you a developer who’s passionate about Mozilla’s mission on the open Web? We need your help: We’re looking for someone to build a game to help keep the Web open as it goes mobile. There’s a Firefox mobile website testing app which we think would make a nice little HTML5 game — with scoring, achievement, levels, leaderboards — and we think you have the chops to make it fun. We’ve had some success making a game of the activity within our team, but we think you could do it better.

Here’s some context. We need lots of people looking at lots of frequently visited websites to see if they look good and work well on mobile. And if they don’t, we need to figure out how to make them better: by finding the bugs and fixing our browser or by working with the developer who built the site to make it work on the open Web. Testing Fennec matters for the future of the open Web:

Fennec, Mozilla’s mobile browser, just landed in Google Play (you may remember it as the Android Marketplace). Firefox Beta for Android is better than ever! You can download it now, use it on your Android phone or tablet and share your feedback. If you get hooked on testing, you’ll want to create a bugzilla account (if you don’t have one) and start filing bugs. By

If you’d like to do a little more directed testing, check out Aaron Train’s most excellent testing app, which sends you to some of the world’s leading websites to share your feedback. This is the app we’d like to gamify, to motivate more people in the Mozilla community to help us keep improving the Firefox mobile web experience for everyone.

What matters is mobile

It’s an interesting exercise to start viewing and interacting with the world’s most frequently used websites in a mobile browser. Any mobile browser. You realize the mobile Web has a ways to go. But there’s more to it than that. David Slater, who leads Product Marketing at Mozilla put it so well that I’m just going to share a note that he sent out internally earlier this week:

The mobile Web is under threat. For 8 years Mozilla has fought to make the Web open on desktop – and won. On mobile, it’s different – the battle is underway. In order to win, we will have to make the Web on mobile devices as compelling for developers and users as native mobile apps are today. Marketplace is about doing that. Boot-to-Gecko, ultimately, is about doing that. But first, we have to break open the mobile web and expose the issues.

Today, Apple and Google – and therefore browsers based on Webkit – are dominating the mobile Web, and as a result developers are coding for a single rendering engine. Like we did with desktop, we have to ensure developers have access to truly open standards. And that means that we need to do whatever it takes to establish Gecko‘s presence on mobile – and specifically, on the Android platform which is widely forecast to grow more than any other in the next 5 years.

There are many ways you can join us in this battle, but if you’ve been wanting to test your skills by building a Web game, there’s never been a better time to try, and never for a better cause. And if you need a little help, there are many places you can ask, like our IRC channel, #devrel, the engagement-developers mailing list, or simply @mozhacks on Twitter. And, of course, if you do, we’ll make you MDN-famous, and treat you like a hero. Thank you.

Heroes Wanted

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Mate USA Group Reversible Pumps, Boat Horns, Rub Rails and Bimini Tops Marketed via New Distributors and Websites

Italian manufacturers leverage strategic marketing agreements and Semantic Web design by Miami Web 3.0 front-end developers to expand their presence in the U.S. boating and marine industry.Ft. Lauderdale/Miami, Florida (PRWEB) July 17, 2011 According to a 22 June 2011 release from the National Marine Manufacturers Association, the dollar value of U.S. wholesale shipments of outboard-, inboard …

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