CSS Outline A Comprehensive Guide for Web Designers

Introduction

The outline CSS property is a shorthand that lets you define multiple outline properties in one go. It’s useful for adding stylish borders around elements without affecting the layout. Outlines are lines that appear outside an element’s border and do not take up space, making them ideal for accessibility features like focus indicators.

Specification

The outline property is defined by the CSS Basic User Interface Module Level 4 (CSS UI Level 4). This specification ensures consistent behavior across different browsers and platforms.

Constituent Properties

The outline property combines several individual outline properties:

1. outline-width:
   • Sets the thickness of the outline.
   • Defaults to medium if not specified.
   • Examples: thin, medium, thick, or any CSS length value (e.g., 2px, 1em).
2. outline-style:
   • Defines the style of the outline.
   • Defaults to none if not specified.
   • Examples: none, dotted, dashed, solid, double, groove, ridge, inset, outset.
3. outline-color:
   • Specifies the color of the outline.
   • Defaults to currentcolor.
   • Examples: red, #ff0000, rgb(255, 0, 0), hsl(0, 100%, 50%).

These properties can be set individually or combined using the outline shorthand. Any omitted values will default to their initial settings.

Syntax

The outline property syntax:

outline: [<'outline-width'> || <'outline-style'> || <'outline-color'>] | initial | inherit | revert | revert-layer | unset;

<outline-width> = <length [0,∞]> | thin | medium | thick;
<outline-style> = auto | <outline-line-style>;
<outline-color> = auto | <color> | <image-1D>;

<image-1D> = <stripes()>;
<stripes()> = stripes( <color-stripe># );
<color-stripe> = <color> && ( <length-percentage> | <flex> )?;
<length-percentage> = <length> | <percentage>;

Values

The outline property accepts various values to customize the appearance:

1. <'outline-width'>:
   • Sets the thickness of the outline.
   • Default Value: medium if absent.
   • Example Values: thin, medium, thick, or any valid CSS length value (e.g., 2px, 1em).
2. <'outline-style'>:
   • Defines the style of the outline.
   • Default Value: none if absent.
   • Example Values: none, dotted, dashed, solid, double, groove, ridge, inset, outset.
3. <'outline-color'>:
   • Specifies the color of the outline.
   • Default Value: currentcolor.
   • Example Values: Any valid CSS color value, such as red, #ff0000, rgb(255, 0, 0), or hsl(0, 100%, 50%).

Examples

Setting the style only:

outline: solid;

Setting the style and color:

outline: dashed #f66;

Setting the width and style:

outline: thick inset;

Setting the width, style, and color:

outline: 3px solid green;

Using global values:

outline: inherit;
outline: initial;
outline: revert;
outline: revert-layer;
outline: unset;

Key Points

• Order of Values: The order does not matter. For example, outline: 3px solid green; is the same as outline: green solid 3px;.
• Default Values: Omitted properties default to initial values.
• Global Values: Accepts global values like initial, inherit, revert, revert-layer, and unset.

Description

An outline is a special type of border that appears outside an element’s actual border and does not take up space in the layout. This makes outlines useful for adding visual emphasis without affecting the document’s structure. Outlines are often used for focus indicators, essential for accessibility.

Accessibility

The outline property is crucial for web accessibility by providing visual focus indicators for interactive elements like links, buttons, and form fields. These indicators are essential for users who navigate using keyboards or assistive technologies.

Best Practices for Accessibility

1. Maintain Visibility: Ensure the outline is visible and contrasts well with the background.
2. Avoid Removing Outlines: If removing the default outline, provide an alternative focus indicator.
3. Use Additional Styles: Enhance visibility with properties like border, background-color, or box-shadow.

Example of Accessible Focus Style

<a href="#">This link has a custom focus style.</a>

a:focus {
  outline: 4px dotted #e73;
  outline-offset: 4px;
  background: #ffa;
}

Formal Definition

The outline property is a shorthand that combines several individual properties:

Initial Value:

• outline-width: medium
• outline-style: none
• outline-color: currentcolor

Applies To: All elements.

Inherited: No.

Computed Value:

• outline-width: Computed value as specified for the width.
• outline-style: Computed value as specified for the style.
• outline-color: Computed value as specified for the color.

Animation Types

• outline-width: Can be animated as a length.
• outline-style: Can be animated by computed value type.
• outline-color: Can be animated as a color.

Browser Compatibility

The outline property is widely supported across major browsers.

Desktop Browsers

• Google Chrome: Full support since version 1.0.
• Mozilla Firefox: Full support since version 1.5.
• Microsoft Edge: Full support since version 12.
• Internet Explorer: Partial support starting from version 8.0.
• Opera: Full support since version 7.0.
• Safari: Full support since version 1.2.

Mobile Browsers

• Chrome for Android: Full support since version 18.
• Firefox for Android: Full support since version 4.0.
• Safari on iOS: Full support since iOS 3.2.
• Opera Mobile: Full support since version 10.
• Samsung Internet: Full support since version 1.0.

Resources for Browser Compatibility

• MDN Web Docs: [CSS outline Property]WebsiteUrl
• Can I Use: [Outline Property]WebsiteUrl

Conclusion

The outline property in CSS is a powerful tool for creating visually appealing and accessible designs. Its wide browser support makes it reliable for modern web development. By understanding its capabilities, you can enhance the user experience while ensuring consistency across different devices and platforms.