The CSS 3
Backgrounds and Borders module introduces
property, which allows you to make the border of any CSS box be a
browsers (such as Firefox and SeaMonkey) have implemented parts of
this feature for some time, as
have Webkit-based browsers
(such as Safari and Chrome). Firefox 3.5 adds support for
elliptical corners, and brings the Gecko implementation into line
with the standard on many details.
At the time of writing, the
Backgrounds and Borders module is still
Working Draft, so all the browsers that
border-radius require you to
property names for it. This article uses the standard property
names in all examples, but there is a table of equivalences at the
end. Mozilla’s policy is to enable the standard names once the
module reaches Candidate Recommendation stage;
|CSS source||In your|
width: 48px; height: 48px; border: 1px solid black; border-radius: 10px;
border-radius: 10px 0px;
border-radius: 10px 0px 20px;
border-radius: 10px 0px 20px 30px;
border-top-left-radius: 20px 30px;
border-radius: 10px 0 / 30px 0;
border-radius: 10px / 30px 30px 10px 10px;
In its simplest form,
border-radius takes a single CSS
<length> and rounds all four corners of a box
border by that amount. Rounded corners are never inherited by
If you want the corners to have different sizes, you can list up to
<length>s; these round individual corners,
counting clockwise from the top left. With two or three numbers,
the missing values default to the value for the diagonally opposite
You can also use
longhand properties to control the
curvature of each corner independently from the
border-bottom-left-radius. Each takes a
<length>. There is no defaulting from one
corner to another with these properties.
The Backgrounds module adds the possibility of elliptical corners.
If you’re using the
longhand properties, you simply add a
<length> value to the property; the first
value sets the horizontal semi-axis of the ellipse, the second sets
the vertical semi-axis. With the
border-radius property, the notation is slightly more
involved: after the first list of up to four
<length>s, you add a forward slash and another
list of up to four
<length>s. The values before
the slash set the horizontal semi-axes, and the values after set
the vertical semi-axes; both go clockwise from top left, and
default independently to diagonally opposite corners. If
either semi-axis of a corner is zero, that corner will not be
Webkit does not
border-radius as a shorthand property; it
accepts only one
<length> value. If you want
any effect other than setting all four corners to the same circular
radius, you must use the longhand properties with Webkit.
|Constant thickness||Varying thickness||Sharp inner corner|
border-radius properties directly control the
horizontal and vertical semi-axes of the outer curve of the
border. The inner curve is determined by reducing each
semi-axis by the thickness of the side border adjacent to that end
of the curve. This keeps the curved portion of the border within a
rectangle whose sides are the semi-axes set by
|Inner curve kink effect|
border: 6px solid black; border-radius: 8px / 20px;
When both sides’ borders are the same thickness, the curve has
constant thickness; when they are not the same, the curved portion
of the border interpolates smoothly between the two. When the
border is at least as thick as the
on at least one side, the inner curve becomes a sharp corner.
Thick elliptical borders sometimes look kinked because the inner
curve is very flat.
|CSS source||In your|
width: 48px; height: 48px; border: 1px solid black; border-radius: 25px 0;
border-radius: 40px 0;
border-radius: 50px 0;
border-radius: 40px 10px 10px 10px;
border-radius: 50px 10px 10px 10px;
border-radius:100px 10px 10px 10px;
When the curves are too big for the box, something has to give. The Backgrounds module constrains the sum of the semi-axes on each side of the box to be no more than the outer dimension of the border box on that side; thus you can make one corner curve all the way to the opposite side of the box, as long as both corners adjacent to it are not curved at all. When the sum is too big on even one side, the renderer is required to compute a reduction factor that will make the largest pair of semi-axes fit exactly within the box, and apply that reduction to all the semi-axes. This has advantages and disadvantages; in the spec’s words,
This [rule] ensures that quarter circles remain quarter circles and large [corners] remain larger than smaller ones, but it may reduce corners that were already small enough, which may make borders of nearby elements that should look the same look different.
At time of writing, this rule is implemented in full only by Firefox 3.5 and very recent versions of Webkit (library version 1.1.8 and later; see bug 8736). Older Gecko-based browsers scale each arc independently, and limit each corner to half the outer dimensions of the border box. Older Webkit-based browsers ignore all the corner radii for a box if the sum of the semi-axes on any side is greater than the outer dimension of the border box on that side.
|Color and style
border-radius: 20px; border: 4px solid; border-color: red green blue gray;
border: 6px black; border-style: solid solid double double;
The Backgrounds module leaves the visual appearance of a transition between border colors and/or border styles undefined, but does require that they take place entirely within the curved region of the border. It does, however, recommend the use of a gradient for transitions involving color only.
Both Gecko- and Webkit-based browsers currently implement all such transitions as abrupt, occurring at the diagonal of the rectangle that encloses the corner curve. This may change in the future, at least for color transitions; see bug 483696 for Gecko.
border-width: 2px; border-radius: 15px; border-style: dotted;
In terms of what gets drawn on the screen, inset and outset borders
are the same as solid borders that aren’t the same color all the
way around, so they are also affected the same way
border-radius. Double, groove, and ridge borders
are drawn with two or three stripes within the border thickness.
The arc-reduction rule described above is applied to each stripe;
this can exaggerate the apparent kink when the innermost arc is too
flat, but otherwise works well.
For dotted and dashed borders, the dots or dashes are supposed to continue through the corner curve. Gecko-based browsers don’t implement this at all; the curved portions of the border are drawn as solid (bug 382721). Webkit draws dots and dashes along the curves, but may show artifacts at the ends of the curves.
border-radius: 15px; border: none; overflow: visible;
Both colored and image backgrounds are clipped to the curved
border. This happens even if there is no visible border
border-style: none, for instance).
is not clipped, but backgrounds are still clipped to the
curve, even though in the presence of
the regular border is not drawn.
Content is, by default, not clipped at all. If you
overflow:hidden, however, the Backgrounds module
requires content to be clipped to the border curve. This works
with very recent Webkit
9543) but not Gecko
459144). or older versions of Webkit.
The spec says
The UA may reduce or treat as zero the
border-radius for a given corner if a scrolling mechanism is
present in that corner. In other words, if there are scrollbars
corners on the sides that actually have scrollbars may not be
rounded. This is because the scrollbars themselves obviously
should not be clipped, and the visual inconsistency was expected to
be undesirable in at least some cases.
By default, the clipping path is the outer rounded
rectangle; thus, the background will show under the border if the
border is not fully opaque. (This is visible in several of the
examples above.) The Backgrounds module includes a
background-clip property that can change this.
Gecko and Webkit both include partial support for this property,
but they conform to an older revision of the
spec: the value keywords are
padding instead of
no-clip are not supported.
using Webkit 1.1.10)
(6, 7, 8)
|border-radius||-moz-border-radius||-webkit-border-radius||Not supported||Not supported|
|More than one circular radius in shorthand||Works||Works||Not supported||Not supported|
|Elliptical curves in shorthand||Works||Not supported||Not supported||Not supported|
|Oversized radius reduction||Works||Buggy||Works||Buggy|
|Dotted and dashed corners||Drawn solid||Drawn solid||Artifacts||Artifacts|