3 posts on border-radius

Preview corner-shape, before implementations!

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As an editor of the Backgrounds & Borders Level 4 spec, I am naturally a bit more interested in the cool features it will bring, once implementations start (it’s currently too early for that). One of the coolest features in it is corner-shape. While in Backgrounds & Borders 3, border-radius was only used for rounded (actually, elliptical) corners, with the help of corner-shape, it will be able to do so much more! Beveled corners, scoop-style corners (informally known as “negative border-radius”), even rectangular notches.

Unfortunately, until it’s implemented in browsers, it’s hard to play with it. Or, is it? I spent the weekend creating an app in which you can enter values for corner-shape, border-radius, width, and height, and see the result, simulated through SVG, as well as the fallback in browsers that don’t support border-corner-radius (which is currently all browsers).

corner-shape preview

Obviously, it’s not a full preview, since you can only play with a limited subset of CSS properties, but it should be good for seeing the kinds of shapes that will be possible.You could also copy the generated SVG from the Developer tools of your browser, and use it as a background in any website!

Use it here: corner-shape preview

Tested to work in at least Chrome, IE9, Firefox, Safari and theoretically, should work in any SVG-enabled browser.

Enjoy! Hope you like it.

Important: Please note that corner-shape is still at a very early stage and might completely change before implementations. You can also help to make it better: Play with it and comment on what you think about its naming and functionality!

The curious case of border-radius:50%

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Admittedly, percentages in border-radius are not one of the most common use cases. Some even consider them an edge case, since most people seem to set border-radius in pixels or --rarely-- ems. And since it’s not used very frequently, it’s still quite buggy. A bit of a chicken and egg case actually: Is it buggy because it’s used rarely or is it used rarely because it’s buggy? My vote would go to the first, so the purpose of this post is to let people know about why percentages in border-radius are incredibly useful and to highlight the various browser whims when it comes to rendering them.


Before we go into discussing implementations, let’s first examine what the right thing to do is, i.e. what the specification says:

Percentages: Refer to corresponding dimension of the border box.

The two length or percentage values of the ‘border-*-radius’ properties define the radii of a quarter ellipse that defines the shape of the corner of the outer border edge (see the diagram below). The first value is the horizontal radius, the second the vertical radius. If the second value is omitted it is copied from the first. If either length is zero, the corner is square, not rounded. Percentages for the horizontal radius refer to the width of the border box, whereas percentages for the vertical radius refer to the height of the border box.

Why is that useful?

It’s the only way of utilizing border-radius to draw a circle or ellipse, i.e. a rounded shape without any straight lines whatsoever (without knowing the dimensions in advance).

As you will see below, Firefox used to have a bug, or actually a different interpretation of the spec, which I think is a quite commonly needed use case, even more than ellipses: It always drew a regular curve for the corners (quarter of a circle) with the maximum possible radii. This is a very commonly needed shape in UI design. If you’re using OSX, you’re seeing it everywhere: the buttons, the scrollbars, even Skype (notice the blue or grey shading around the usernames in a chat). As I’m writing this post, I can see the same shape in the buttons of Wordpress’ admin panel. And as the current spec stands, there’s no way to do that. You have to know the height (or width, if you want a vertical shape) in advance, which even when possible, makes border-radius depend on the value of other attributes (such as line-height) and you have to remember to change it every time you change those, which causes maintenance headaches. And what’s worse is that the Backgrounds & Borders module is almost done, so it’s quite unlikely that this will change anytime soon. :(

As noted in this comment by David Baron, that assumption wasn’t exactly correct about Firefox’s old rendering. It just resolved % as relative to width in every case (kinda like percentages in margins) and when the height was smaller than the width, it applied the rules for radii that are too big, which say to reduce it equally. A straightforward deduction is that we do have a standards-compliant way to get the behavior from old versions of Firefox, in every browser: Just specify a very big radius, like 9999px.

Different implementations, different bugs

Firefox 4 beta 6

As I mentioned above, Gecko up to Firefox version 4 beta 6 always draws a regular curve for the corners with the largest radii applicable, resulting in a shape that is either a perfect circle or a rectangle with a semicircle on top and bottom (if height > width) or right and left (if width > height).

Minefield (latest Gecko nightlies)

In the latest nightlies this bug is fixed, and it follows the spec to the letter. I can’t help but wonder if this was a bug, a misinterpretation of the spec or a deliberate disagreement with it.

WebKit nightlies

Webkit was late to support percentages in border-radius, but it seems to be the first (it or IE9, I’m not sure) to follow the spec to the letter --concerning corner radii at least-- and renders an ellipse (horizontal radius = width/2, vertical radius = height/2) no matter what. Webkit however seems to be having serious trouble with borders, rendering them with variable width strokes (!).

Opera 11

Presto (Opera) is the weirdest when it comes to rendering a percentage border-radius. I can’t figure out the algorithm it uses to determine the radii of the corners even if it was to save my life, it even changes according to window size in my testcases! Since I’ve been using border-radius:50% regularly, I’ve had the pleasure of observing Opera’s rendering in many different designs and I still can’t find a pattern. It’s particularly funny when rendering the little fuchsia comment bubbles in the homepage of my blog: Every one of them has a different radius, even if they are about the same size. It even got one of them right and rendered it as an ellipse once!

Internet Explorer 9

Trident (IE9), along with the latest Gecko nightly is the only 100% correct one when it comes to rendering the testcases, which is not surprising since the IE team boasted quite a lot for their bulletproof border-radius implementation. Well, their CSS3 support might be a bit lacking, but at least the bits they actually implement aren’t buggy. Kudos for that.

Link to testcases

Note: Of course all bugs mentioned above have been reported to the respective browser vendors (except the Gecko one that is already fixed in the nightlies).

CSS3 border-radius, today

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This is the first one from a series of articles I’m going to write about using CSS3 properties or values today. I’ll cover everything I have found out while using them, including various browser quirks and bugs I know of or have personally filed regarding them. In this part I’ll discuss ways to create rounded corners without images and if possible without JavaScript in the most cross-browser fashion.

I will not cover irregular curves in this article, since I’ve yet to find any person who actually needed them, even once, including myself and browser support for them is far worse.

Caution: The contents of a container with border-radius set are NOT clipped according to the border radius in any implementation/workaround mentioned below, and no, setting overflow to hidden won’t help (and even if it did, you’d risk text missing). You should specify a proper border-radius and/or padding to them if you want them to  follow their container’s curves properly. This could allow for some nice effects but most of the time it’s just a pain in the a$$.

Mozilla Firefox

Firefox supports rounded corners since version 2. However incomplete support in version 2 made designers sceptical to use them. The problem was that the rounded corners created were aliased back then, and also did not crop the background image, so if you had one, no rounded corners for you. This was fixed in FF3, so now more and more designers are starting to use them. The syntax is

-moz-border-radius: [Number][unit];

This is effectively a shorthand for:

-moz-border-radius-bottomleft: [Number][unit];
-moz-border-radius-bottomright: [Number][unit];
-moz-border-radius-topleft: [Number][unit];
-moz-border-radius-topright: [Number][unit];

You don’t need to specify all these properties though, even if you wan’t different measures per corner, as -moz-border-radius functions as a regular CSS shorthand, allowing us to specify all 4 corners at once. It can be used in the following ways:

-moz-border-radius: [Top-left and Bottom-right] [Top-right and bottom-left];
-moz-border-radius: [Top-left] [Top-right and bottom-left] [Bottom-right];
-moz-border-radius: [Top-left] [Top-right] [Bottom-right] [Bottom-left];

A good mnemonic rule for the order of the values is that they are arranged clockwise, starting from Top left.

Apple Safari

Safari also implements CSS3 border-radius, but in a quite different way. If you want to set all four corners to the same border-radius, the process is almost identical. The only thing needed is:

-webkit-border-radius: [Number][unit]

However, things start to get tricky when you want to specify different radiuses per corner. Webkit does not support a shorthand syntax, since it chose to implement the spec closely, sacrifycing clarity but allowing for more flexibility. To cut a long story short, Webkit supports irregular curves instead of just circle quarters on each corner, so if you try to add 2 values, the result will be  horrendous.

So, you have to specify all four properties (or less if you want some of them to be square). To make matters even worse, the way the names of the properties are structured is different. There is one more dash, and the position of the corner styled by each property is not at the end but before -radius:


Caution: If the dimensions of your element are not enough to accomodate the rounded corners, they will be square in Webkit-based browsers. Specify a min-width/min-height or enough padding to avoid this.

Google Chrome

Since Google Chrome is based on Webkit, its border-radius support is like Safari’s. However, it’s haunted by an ugly bug: It renders the rounded corners aliased. :(


The bad news is that Opera does not implement the CSS3 border-radius yet (it will in the future, confirmed). The good news is that it allows for SVG backgrounds since version 9.5. The even better news is that it supports data:// URIs, so you can embed the SVG in your CSS, without resorting to external files as someone recently pointed out to me. Alexis Deveria was clever enough to even create a generator for them, so that you could easily specify the background, border width and border-color and get the data URI instantly. This is a quite useful tool, but lacks some features (for instance you might want the background to be semi-transparent, like the one used in this blog). It’s ok for most cases though.

While Opera’s current lack of border-radius support is disappointing, you can utilize it pretty well with this method and if you know SVG well enough yourself you can create stunning effects.

Internet Explorer (aka “The Web designer’s nemesis”)

There’s no need to tell you that IE doesn’t support border-radius or SVG backgrounds, even in it’s latest version, right? You probably guessed already. There is some hope here though, a clever guy named Drew Diller carefully researched the MS-proprietary VML language and came up with a script that utilizes it to create rounded corners in IE. The bad news is that MS when releasing IE8 fixed some things and messed up others, so the script barely works on it. It also has some other shortcomings, but for most cases it can be a great tool (for IE7 and below, unless MS surprises us and fixes the VML regressions in IE8 before the stable). Also, if rounded corners are not crucial to your design and you don’t get too much traffic from IE users, you might consider ignoring IE altogether and having square corners in it. This way you’re also serving the greater good, since when IE users see your site in a supporting browser, they’ll conclude that “Oh, this browser shows the web nicer!” and the site will still be just as usable (in most cases rounded corners are not that crucial for usability, although they enchance it a bit).


I hope this article helped you learn something new. If you found any mistakes or inaccuracies, don’t hesitate to leave a comment, I don’t know everything and I’m not god. :)

One thing I have in mind is creating a PHP script that takes care of all these incompatibilities for you and caches the result. I don’t know if I’ll ever find the time to write it though, especially before someone else does :P