C28: Specifying the size of text containers using em units | WAI | W3C Skip to content

Technique C28:Specifying the size of text containers using em units

About this Technique

This technique relates to:

  • 1.4.4: Resize Text (Sufficient when used for Ensuring that text containers resize when the text resizes AND using measurements that are relative to other measurements in the content)
  • 1.4.12: Text Spacing (Advisory)

This technique applies to CSS.

Description

The objective of this technique is to specify the width and/or height of containers, that contain text or that will accept text input, in em units. This will allow user agents that support text resizing to resize the text containers in line with changes in text size settings.

The width and/or height of text containers that have been specified using other units risk text cropping because it falls outside the container boundaries when the text size has been increased.

The containers generally control the placement of text within the Web page and can include layout elements, structural elements and form controls.

Note

This technique uses em units, but using rem units instead would have the same effect.

Examples

Example 1: em units for sizes for layout container containing text

In this example, a div element, with id value of "nav_menu", is used to position the navigation menu along the left-hand side of the main content area of the Web page. The navigation menu consists of a list of text links, with id value of "nav_list". The text size for the navigation links and the width of the container are specified in em units.

#nav_menu { width: 20em; height: 100em; }
#nav_list { font-size: 100%; }

Example 2: em units for text-based form controls

In this example, input elements that contain text or accept text input by the user have been given the class name "form1". CSS rules are used to define the font size in percent units and width for these elements in em units. This will allow the text within the form control to resize in response to changes in text size settings without being cropped (because the width of the form control itself also resizes according to the font size). 

input.form1 { font-size: 100%; width: 15em; }

Example 3: em units in dropdown boxes

In this example, select elements have been given the class name "pick." CSS rules are used to define the font size in percent units and width in em units. This will allow the text within the form control to resize in response to changes in text size settings without being cropped. 

select.pick { font-size: 100%; width: 10em; }

Example 4: em units for non-text-based form controls

In this example, input elements that define checkboxes or radio buttons have been given the class name "choose." CSS rules are used to define the width and height for these elements in em units. This will allow the form control to resize in response to changes in text size settings.

input.choose { width: 1.2em; height: 1.2em; }

Tests

Procedure

  • Identify containers that contain text or allow text input.
  • Check the container's width and/or height are specified in em units.

Expected Results

  • Check #2 is true.
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