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  1. Boxes with Multiple Backgrounds
  2. Borders Consisting of Images / Borders with Rounded Corners
  3. Drop Shadows / Box Decoration Breaks
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Borders Consisting of Images

Elements can have style-based (e.g., double line) or image-based (new in CSS 3) borders. An image-based border's design is taken from the sides and corners of a specified image. Its pieces may be sliced, scaled, and stretched to fit the size of the border image area.

CSS 3 provides the following border image properties:

  • border-image specifies all border image properties in a single declaration.
  • border-image-outset specifies the amount by which the border image area extends beyond the border box.
  • border-image-repeat specifies whether the border image should be repeated, rounded, or stretched.
  • border-image-slice specifies the inward offsets of the border image.
  • border-image-source specifies an image to be used for the border.
  • border-image-width specifies the widths of the border image.

The border-image property is a shorthand property for setting the other border image properties. This property has the following minimal syntax:

border-image: <border-image-source> <border-image-slice> <border-image-repeat>

The border-image-source property specifies an image to use instead of a border style (given by the border-style property that predates CSS 3). This property has the following syntax:

border-image-source: <image> | none

<image> is an image URI (e.g., url(border.png)). If the value is none or if the image cannot be displayed (or if the property doesn't apply), a border style will be used. Otherwise, the element's borders are invisible, and the border image is drawn.

Figure 6 shows a sample border image.

Figure 6 The border image is divided into nine regions

A border image is divided into nine regions: four corners, four edges, and a middle region. The border-image-slice property lets you define these regions by specifying inward offsets from the upper-left corner of the image. This property has the following syntax:

border-image-slice: [<number> | <percentage>]{1,4} && fill?

You can specify four numeric values (representing raster image pixels or vector image coordinates) or percentage values for these offsets. These values are specified in top, right, bottom, and left side order, as in border-image-slice: top right bottom left. See Figure 7.

Figure 7 You can define all nine regions by specifying top, right, bottom, and left offsets

If the left value is missing, it's treated as being equal to the right value. If the bottom value is missing, it's treated as being equal to the top value. If the right value is missing, it's treated as being equal to the top value.

When present, the fill keyword causes the middle part of the border image to be preserved (and drawn over the background)—it's discarded by default.

For example, using the Figure 6 border image (which has a width of 63 pixels and a height of 63 pixels), I could specify border-image-slice: 21 21 21 21 or border-image-slice: 21 to define nine equal-size 21-pixel regions. The middle area is discarded.

The border-image-repeat property specifies how the images for the sides and the middle part of the border image are scaled and tiled. This property has the following syntax:

border-image-repeat: [ stretch | repeat | round | space ]{1,2}

This syntax lets you specify one or two of the following keywords:

  • stretch: Stretch the image to fill the area.
  • repeat: Repeat (tile) the image to fill the area.
  • round: Tile the image to fill the area. If the image doesn't fill the area with a whole number of tiles, rescale the image so that it does.
  • space: Tile the image to fill the area. If the image doesn't fill the area with a whole number of tiles, distribute the extra space around the tiles.

The first keyword applies to the horizontal sides; the second keyword applies to the vertical sides. If the second keyword is absent, it's assumed to be the same as the first keyword.

Listing 5 demonstrates border-image.

Listing 5—Demonstrating the border-image property

<!DOCTYPE html>
<html>
  <head>
    <title>
      border-image demo
    </title>
  </head>

  <body>
    <div style="border: double orange 1em;
                border-image: url(border.png) 21 round stretch; 
                -o-border-image: url(border.png) 21 round stretch;
                -webkit-border-image: url(border.png) 21 round stretch;
                text-align: center; width: 50%;">
      But soft! What light through yonder window breaks?<br>
      It is the East, and Juliet is the sun!<br>
      Arise, fair sun, and kill the envious moon,<br>
      Who is already sick and pale with grief<br>
      That thou her maid art far more fair than she.<br>
      Be not her maid, since she is envious.<br>
      Her vestal livery is but sick and green,<br>
      And none but fools do wear it. Cast it off.<br>
      It is my lady; O, it is my love!<br>
      O that she knew she were!<br>
      She speaks, yet she says nothing. What of that?<br>
      Her eye discourses; I will answer it.<br>
      I am too bold; 'tis not to me she speaks.<br>
      Two of the fairest stars in all the heaven,<br>
      Having some business, do entreat her eyes<br>
      To twinkle in their spheres till they return.<br>
      What if her eyes were there, they in her head?<br>
      The brightness of her cheek would shame those stars<br>
      As daylight doth a lamp; her eyes in heaven<br>
      Would through the airy region stream so bright<br>
      That birds would sing and think it were not night.<br>
      See how she leans her cheek upon her hand!<br>
      O that I were a glove upon that hand,<br>
      That I might touch that cheek!
    </div>
  </body>
</html>

You can interpret border-image: url(border.png) 21 round stretch; as follows:

  • url(border.png) identifies the image shown in Figure 6 as the border image.
  • 21 identifies 21 pixels as the inward offset from the upper-left corner of the image. Each of the nine regions has a 21-pixel width and a 21-pixel height.
  • round stretch tiles the image horizontally and vertically. For horizontal tiling, the image is rescaled (when necessary) so that a whole number of tiles fills the area. For vertical tiling, the image is stretched.

Figure 8 shows an image border for a <div> element that's given Figure 6's border in the context of the Safari browser.

Figure 8 The image border accents a famous monologue from Romeo and Juliet

If you attempt to view this page with Internet Explorer 9, you'll see an orange double-line border because that version of Internet Explorer doesn't support border-image.

Borders with Rounded Corners

CSS 3 lets you give elements rounded borders without having to resort to corner images or multiple <div> elements. Each corner is defined by the radii of a quarter ellipse, as shown in Figure 9.

Figure 9 A corner's curvature is defined by horizontal and vertical radii

For each corner, you can specify one or two values. The first value (a) refers to the horizontal radius and the second value (b) refers to the vertical radius. If you specify only a single value, that value is used for the horizontal and vertical radii. (If a value is zero, the corner is square.)

You can specify the value as a number (e.g., 20px) or as a percentage (e.g., 10%). Percentages for the horizontal radius refer to the width of the border box; percentages for the vertical radius refer to the height of the border box. Negative values are not allowed.

CSS 3 provides the following border radius properties:

  • border-radius specifies all border radius properties in one declaration.
  • border-top-left-radius specifies the radii for the top-left corner.
  • border-top-right-radius specifies the radii for the top-right corner.
  • border-bottom-left-radius specifies the radii for the bottom-left corner.
  • border-bottom-right-radius specifies the radii for the bottom-right corner.

Listing 6 demonstrates border-radius, border-top-left-radius, and border-bottom-right-radius.

Listing 6—Demonstrating the border-radius, border-top-left-radius, and border-bottom-right-radius properties

<!DOCTYPE html>
<html>
  <head>
    <title>
      border-radius demo
    </title>
  </head>

  <body>
    <div style="border: solid black 1em;
                border-radius: 20px;
                width: 100px; height: 100px;
                float: left; margin-right: 20px;">
    </div>

    <div style="border: solid red 1em;
                border-radius: 20%;
                width: 100px; height: 100px;
                float: left; margin-right: 20px;">
    </div>

    <div style="border: solid green 1em;
                border-top-left-radius: 10px 20px;
                width: 100px; height: 100px;
                float: left; margin-right: 20px;">
    </div>

    <div style="border: solid blue 1em;
                border-top-left-radius: 15px 30px;
                border-bottom-right-radius: 30px; 15px;
                width: 100px; height: 100px;
                float: left; margin-right: 20px;">
    </div>
  </body>
</html>

Figure 10 shows the resulting rounded borders in the context of the Firefox browser.

Figure 10 The border radius properties apply to styled borders only. They don't also apply to image borders

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