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The CSS Pocket Guide: Measurements, URLs, and Color Units

Nov 17, 2010

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Color with Alpha Transparency

CSS3 defines the ability to add a level of transparency to the otherwise solid color designation. Applying alpha transparency to borders, text colors, or backgrounds allows the color of the elements behind the targeted element to bleed through or combine with what is behind it. Figure 4.1 shows the use of a transparent background color to mute the distractions created by a background image so that text can be readable.

Figure 4.1 Using a transparent background color to make solid text more readable against a background image.

The transparency in the color applies only to the parts of the element that color applies to and does not affect the transparency of elements it may contain or other objects like images. I discuss the opacity property, which applies to the visibility of an entire element, in Chapter 6.

`rgba(r,g,b,a)`

The r, g, and b values work on the same scale as their rgb() unit counterparts discussed earlier in this chapter. The alpha (a) value is a decimal number from 0 to 1, with 0 being completely transparent and 1 being fully opaque.

section { background-color: rgba(0,200,0,0.1); } /* very
   transparent green */

`hsla(h,s,l,a)`

Like rgba(), hsla() is the same color as hsl() with an added designation for alpha transparency. This unit is also a decimal number from 0 to 1.

a { background-color: rgba(0,0,0,0.9); } /* almost solid
   black */

`transparent`

The transparent color keyword represents a color value that is fully transparent (and thus red channel or hue designations don't matter). You can think of it as a shorthand form for (and gets computed in browsers as) rgba(0,0,0,0).

rgba() and hsla() Colors with Non-CSS3 Browsers

Colors units with alpha channels are supported in recent versions of Safari, Firefox, Opera, and the upcoming IE9. Browsers that don't support these units will skip them entirely because the rgba() or hsla() notation is as foreign to them as madeupscheme() is. This means if you defined an element's background to be "blue but a little transparent," the browser will not fall back to "blue but not transparent" and instead use whatever the previously defined or inherited color may be. If you want the fallback to be "blue but not transparent," you can write the rule set as follows:

div {
   background-color: rgb(0,0,255);
   background-color: rgba(0,0,255,90%);
}

All browsers will read the first background-color declaration. Browsers with rgba() support will then override that setting with the second declaration, while others will ignore it as invalid syntax.

As a variation on this method, if your layout calls for transparent red against white, resulting in a pink color, you might use that resulting color in the first declaration instead of a deeper red.

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