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13.3 Outlines
 
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13.3 Outlines

the flow of the document like borders do, and thus don't trigger document reflow as they appear and disappear. If you give an element a 50-pixel outline, the outline will very likely overlap other elements. Second, outlines can be nonrectangular—but don't start leaping for joy just yet. This does not mean that you can create circular outlines. Instead, it means that an outline on an inline element may not act like a border would on that same element. With an outline, a user agent is allowed to "merge" the pieces of the outline to create a single continuous, but nonrectangular, shape. Figure 13-10 shows an example.

Figure 13-10. Outlines can have irregular shapes
figs/css2_1310.gif

make sure that outlines do not take up layout space.

There is one other basic way in which outlines and borders differ: they aren't the same thing, and so they can both exist on the same element. This can lead to some interesting effects, such as that illustrated in Figure 13-11.

Figure 13-11. The coexistence of borders and outlines
figs/css2_1311.gif

In the CSS2 specification, there is the following sentence: "The outline may be drawn starting just outside the border edge." Note the word may in that sentence. User agents are encouraged to do as the sentence suggests, but it isn't a requirement. A user agent could decide to draw borders inside the inner border edge or at some small distance from the border. As of this writing, all browsers that support outlines draw them just outside the outer border edge, so, thankfully, there is consistency.

Outlines are considered to be part of user interface styling because they are most often used to indicate the current focus. If a user is using keyboard navigation to jump from link to link, then the link that is currently in focus will usually get an outline. In Internet Explorer for Windows, an outline is applied to any link that has been selected by the user ("clicked," if she's using a mouse). Other browsers apply outlines to text inputs that have the keyboard focus, thus giving a cue regarding where input will go if the user starts typing.

As you'll see, outlines are styled a lot like borders, but there are some key differences besides the ones previously mentioned. We'll just skip quickly over the similarities and spend time looking at the differences.

13.3.1 Setting an Outline's Style

The most basic aspect of an outline, like a border, is its style, which is set using outline-style.

outline-style


Values

none | dotted | dashed | solid | double | groove | ridge | inset | outset | inherit


Initial value

none


Applies to

all elements


Inherited

no


Computed value

as specified


The list of style keywords is largely the same as the keywords for border styles, and the visual effects are the same. There is an omission: hidden is not a valid outline style, and user agents are required to effectively treat it as none. This actually makes sense, given that outlines don't affect layout even when they're visible.

The other difference between outlines and borders is that you can specify only one keyword for an outline-style value (compared with up to four keywords for borders). The practical effect is that outlines must have the same outline style all the way around an element, whether they're rectangular or not. This is probably just as well, since trying to figure out how to apply different styles to the same nonrectangular outline would be a pain.

13.3.2 Outline Width

Once you've brought an outline into being by giving it a style, it's a good idea to use outline-width to define (you guessed it) the outline's width.

outline-width


Values

thin | medium | thick | <length> | inherit


Initial value

medium


Applies to

all elements


Inherited

no


Computed value

absolute length; 0 if the style of the border is none or hidden


The list of keywords should look very familiar to anyone who's set a border width. The only real difference between outline-width and border-width is that, as with the style, you can declare only a single width for the entire outline. Thus, only one keyword is permitted in a value.

13.3.3 Coloring an Outline

Since you can set style and width, it makes sense that outline-color exists to let you give the outline a color.

outline-color


Values

<color> | invert | inherit


Initial value

invert (or user agent-specific; see text)


Applies to

all elements


Inherited

no


Computed value

as specified


Herein lies the most intriguing difference between borders and outlines: the keyword invert, which is the default value. An inverting outline means that a color inversion is performed for the pixels where the outline exists. See Figure 13-12.

Figure 13-12. Inverting color with an outline
figs/css2_1312.gif

then it should use instead the computed value of color for the element.

portions of your document. This isn't really the purpose of outlines, but Figure 13-13 shows one such result anyway.

Figure 13-13. Massive inversion
figs/css2_1313.gif

Of course, if you'd rather define a specific color for your outline, just use any valid color value. The results of the following declarations should be obvious enough:

outline-color: red;

outline-color: #000;

outline-color: rgb(50%,50%,50%);

The potential drawback here is the possibility that an outline color could closely match the colors of the pixels around it, in which case the user won't be able to see it. This is why invert was defined.

As with outline styles and widths, you can define only one color for the entire outline.

13.3.4 Bringing It All Together

Like border for borders, outline is the shorthand property that allows you to set the style, width, and color of an outline all at once.

outline


Values

[ <outline-color> || <outline-style> || <outline-width> ] | inherit


Initial value

not defined for shorthand properties


Applies to

all elements


Inherited

no


Computed value

see individual properties (outline-color, etc.)


As with other shorthands, outline example, the outline will use the color keyword invert since it's implied by the second declaration:

a:focus {outline-color: red; outline: thick solid;}

Because a given outline must be of a uniform style, width, and color, outline is the only shorthand property related to outlines. There are no properties such as outline-top or outline-right.

drawn "on top" of the margin, it will fill in some of that space, as illustrated in Figure 13-14:

div#callbox {outline: 5px solid invert; margin: 5px;}

input:focus {outline: 1em double gray;}
Figure 13-14. Different outlines
figs/css2_1314.gif

If an author uses borders to indicate the focus, then the document layout may shift or jump around. Outlines can yield the same effects borders allow, but without the jumpiness.

the margins (which are transparent), this is not a big issue. If a future version of CSS allows outlines to move inward to overlap the borders or other visible portions of the element box, then the placement of outlines will become more important.

obscured by other elements. You can combine both of these in a single example:

div#one {outline: 1em solid invert;}

div#two {outline: 1em solid invert; margin: -2em -2em 0 2em;

  background: white;}

Now suppose div#two immediately follows div#one in a document. It will overlap the first div, and its background will overlap portions of the first div's outline. I haven't included a figure to go along with this code block because the CSS2 specification doesn't provide any ideas about what would happen. Should the first div's outline be visible, overlapping the background and contents of the second div inverted once and then left unchanged? We don't know. Any illustration here would be neither right nor wrong, but simply a possible outcome—and not necessarily the one user agents end up implementing or the one a future version of CSS defines.

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