Visual effects

# 11 Visual effects

Contents

## 11.1 Overflow and clipping

Generally, the content of a block box is confined to the content edges of the box. In certain cases, a box may overflow, meaning its content lies partly or entirely outside of the box, e.g.:

- A line cannot be broken, causing the line box to be wider than the block box.
- A block-level box is too wide for the containing block. This may happen when an element's 'width' property has a value that causes the generated block box to spill over sides of the containing block.
- An element's height exceeds an explicit height assigned to the containing block (i.e., the containing block's height is determined by the 'height' property, not by content height).
- A descendent box is positioned absolutely, partly outside the box. Such boxes are not clipped by the overflow property on their ancestors.
- A descendent box has negative margins, causing it to be positioned partly outside the box.
- The 'text-indent' property causes an inline box to hang off either the left or right edge of the block box.

Whenever overflow occurs, the 'overflow' property specifies whether a box is clipped to its content box, and if so, whether a scrolling mechanism is provided to access any clipped out content.

### 11.1.1 Overflow: the 'overflow' property

'overflow'

  Value: | visible | hidden | scroll | auto | inherit
  Initial: | visible
  Applies to: | block-level and replaced elements, table cells, inline blocks
  Inherited: | no
  Percentages: | N/A
  Media: | visual
  Computed value: | as specified

This property specifies whether content of a block-level element is clipped when it overflows the element's box. It affects the clipping of all of the element's content except any descendant elements (and their respective content and descendants) whose containing block is the viewport or an ancestor of the element. Values have the following meanings:

visible
  This value indicates that content is not clipped, i.e., it may be rendered outside the block box.
hidden
  This value indicates that the content is clipped and that no scrolling user interface should be provided to view the content outside the clipping region; users will not have access to clipped content.
scroll
  This value indicates that the content is clipped and that if the user agent uses a scrolling mechanism that is visible on the screen (such as a scroll bar or a panner), that mechanism should be displayed for a box whether or not any of its content is clipped. This avoids any problem with scrollbars appearing and disappearing in a dynamic environment. When this value is specified and the target medium is 'print', overflowing content may be printed.
auto
  The behavior of the 'auto' value is user agent-dependent, but should cause a scrolling mechanism to be provided for overflowing boxes.

Even if 'overflow' is set to 'visible', content may be clipped to a UA's document window by the native operating environment.

HTML UAs may apply the overflow property from the BODY or HTML elements to the viewport.

In the case of a scrollbar being placed on an edge of the element's box, it should be inserted between the inner border edge and the outer padding edge.

Example(s):

Consider the following example of a block quotation (`<blockquote>`) that is too big for its containing block (established by a `<div>`). Here is the source document:

    <div>
    <blockquote>
    <p>I didn't like the play, but then I saw
    it under adverse conditions - the curtain was up.</p>
    <cite>- Groucho Marx</cite>
    </blockquote>
    </div>

Here is the style sheet controlling the sizes and style of the generated boxes:

    div { width : 100px; height: 100px;
          border: thin solid red;
          }

    blockquote   { width : 125px; height : 100px;
          margin-top: 50px; margin-left: 50px;
          border: thin dashed black
          }

    cite { display: block;
           text-align : right;
           border: none
           }

The initial value of 'overflow' is 'visible', so the `<blockquote>` would be formatted without clipping, something like this:

Setting 'overflow' to 'hidden' for the `<div>`, on the other hand, causes the `<blockquote>` to be clipped by the containing block:

A value of 'scroll' would tell UAs that support a visible scrolling mechanism to display one so that users could access the clipped content.

Finally, consider this case where an absolutely positioned element is mixed with an overflow parent.

Stylesheet:

      container { position: relative; border: solid; }
      scroller { overflow: scroll; height: 5em; margin: 5em; }
      satellite { position: absolute; top: 0; }
      body { height: 10em; }

Document fragment:

      <container>
       <scroller>
        <satellite/>
        <body/>
       </scroller>
      </container>

In this example, the "scroller" element will not scroll the "satellite" element, because the latter's containing block is outside the element whose overflow is being clipped and scrolled.

### 11.1.2 Clipping: the 'clip' property

A clipping region defines what portion of an element's border box is visible. By default, the clipping region has the same size and shape as the element's border box. However, the clipping region may be modified by the 'clip' property.

'clip'

  Value: | <shape> | auto | inherit
  Initial: | auto
  Applies to: | absolutely positioned elements
  Inherited: | no
  Percentages: | N/A
  Media: | visual
  Computed value: | For rectangle values, a rectangle consisting of four computed lengths; otherwise, as specified

The 'clip' property applies only to absolutely positioned elements. Values have the following meanings:

auto
  The element does not clip.
<shape>
  In CSS 2.1, the only valid <shape> value is: rect(<top>, <right>, <bottom>, <left>) where <top> and <bottom> specify offsets from the top border edge of the box, and <right>, and <left> specify offsets from the left border edge of the box in left-to-right text and from the right border edge of the box in right-to-left text. Authors should separate offset values with commas. User agents must support separation with commas, but may also support separation without commas, because a previous version of this specification was ambiguous in this respect.

  <top>, <right>, <bottom>, and <left> may either have a <length> value or 'auto'. Negative lengths are permitted. The value 'auto' means that a given edge of the clipping region will be the same as the edge of the element's generated border box (i.e., 'auto' means the same as '0' for <top> and <left> (in left-to-right text, <right> in right-to-left text), the same as the computed value of the height plus the sum of vertical padding and border widths for <bottom>, and the same as the computed value of the width plus the sum of the horizontal padding and border widths for <right> (in left-to-right text, <left> in right-to-left text), such that four 'auto' values result in the clipping region being the same as the element's border box).

  When coordinates are rounded to pixel coordinates, care should be taken that no pixels remain visible when <left> and <right> have the same value (or <top> and <bottom> have the same value), and conversely that no pixels within the element's border box remain hidden when these values are 'auto'.

An element's clipping region clips out any aspect of the element (e.g. content, children, background, borders, text decoration, outline and visible scrolling mechanism — if any) that is outside the clipping region.

The element's ancestors may also clip portions of their content (e.g. via their own 'clip' property and/or if their 'overflow' property is not 'visible'); what is rendered is the cumulative intersection.

If the clipping region exceeds the bounds of the UA's document window, content may be clipped to that window by the native operating environment.

Example(s):

The following two rules:

    p { clip: rect(5px, 40px, 45px, 5px); }
    p { clip: rect(5px, 55px, 45px, 5px); }

will create the rectangular clipping regions delimited by the dashed lines in the following illustrations:

Note. In CSS 2.1, all clipping regions are rectangular. We anticipate future extensions to permit non-rectangular clipping. Future versions may also reintroduce a syntax for offsetting shapes from each edge instead of offsetting from a point.

## 11.2 Visibility: the 'visibility' property

'visibility'

  Value: | visible | hidden | collapse | inherit
  Initial: | visible
  Applies to: | all elements
  Inherited: | yes
  Percentages: | N/A
  Media: | visual
  Computed value: | as specified

The 'visibility' property specifies whether the boxes generated by an element are rendered. Invisible boxes still affect layout (set the 'display' property to 'none' to suppress box generation altogether). Values have the following meanings:

visible
  The generated box is visible.
hidden
  The generated box is invisible (fully transparent, nothing is drawn), but still affects layout. Furthermore, descendents of the element will be visible if they have 'visibility: visible'.
collapse
  Please consult the section on dynamic row and column effects in tables. If used on elements other than rows, row groups, columns, or column groups, 'collapse' has the same meaning as 'hidden'.

This property may be used in conjunction with scripts to create dynamic effects.

In the following example, pressing either form button invokes a user-defined script function that causes the corresponding box to become visible and the other to be hidden. Since these boxes have the same size and position, the effect is that one replaces the other. (The script code is in a hypothetical script language. It may or may not have any effect in a CSS-capable UA.)

    <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
     "http://www.w3.org/TR/1999/REC-html401-19991224/strict.dtd">
    <HTML>
    <HEAD><TITLE>Dynamic visibility example</TITLE>
    <META
     http-equiv="Content-Script-Type"
     content="application/x-hypothetical-scripting-language">
    <STYLE type="text/css">
    <!--
       #container1 { position: absolute;
                     top: 2in; left: 2in; width: 2in }
       #container2 { position: absolute;
                     top: 2in; left: 2in; width: 2in;
                     visibility: hidden; }
    -->
    </STYLE>
    </HEAD>
    <BODY>
    <P>Choose a suspect:</P>
    <DIV id="container1">
       <IMG alt="Al Capone"
            width="100" height="100"
            src="suspect1.png">
       <P>Name: Al Capone</P>
       <P>Residence: Chicago</P>
    </DIV>

    <DIV id="container2">
       <IMG alt="Lucky Luciano"
            width="100" height="100"
            src="suspect2.png">
       <P>Name: Lucky Luciano</P>
       <P>Residence: New York</P>
    </DIV>

    <FORM method="post"
          action="http://www.suspect.org/process-bums">
       <P>
       <INPUT name="Capone" type="button"
              value="Capone"
              onclick='show("container1");hide("container2")'>
       <INPUT name="Luciano" type="button"
              value="Luciano"
              onclick='show("container2");hide("container1")'>
    </FORM>
    </BODY>
    </HTML>