CSS Custom Highlight Priority: Text Selection Layer Management for Global Accessibility

The web is a global platform, and ensuring a consistent and accessible user experience for everyone, regardless of their language, location, or device, is paramount. One often overlooked aspect of user experience is text selection. While seemingly simple, the text selection layer can be customized using CSS to provide better visual cues, improved accessibility, and even enhanced functionality. This blog post explores the CSS Custom Highlight API, focusing on how to control text selection layer priority and manage highlights for global accessibility.

Understanding the Text Selection Layer

When a user selects text on a webpage, the browser applies a default highlight, typically a blue background with white text. This highlight is controlled by the ::selection pseudo-element. However, with the advent of CSS Houdini and the Custom Highlight API, developers now have much greater control over how text is highlighted, including the ability to define multiple highlight layers and control their priority.

The text selection layer is essentially a visual layer rendered on top of the normal content flow. It allows you to customize the appearance of selected text and other highlighted regions. Understanding how this layer interacts with other CSS properties is crucial for creating visually appealing and accessible web experiences.

Introducing the CSS Custom Highlight API

The CSS Custom Highlight API is a part of the CSS Houdini suite of APIs that enable developers to extend CSS functionality. It provides a way to define custom highlights using the ::highlight pseudo-element and the CSS.registerProperty() method. This allows for more sophisticated and flexible text highlighting, going beyond the basic ::selection styling.

Key Concepts:

• ::highlight(highlight-name): This pseudo-element targets a specific custom highlight named highlight-name. You need to register the highlight name first.
• CSS.registerProperty(): This method registers a new custom property, including its syntax, inheritance behavior, initial value, and the custom highlight name it’s associated with.
• Highlight Painter: A custom painter that determines how the highlight should be rendered (e.g., adding a gradient, an image, or a more complex visual effect). This often involves using the CSS Painting API.

Controlling Highlight Priority

One of the most powerful features of the Custom Highlight API is the ability to control the priority of different highlight layers. This is crucial when you have multiple overlapping highlights and need to determine which highlight should be visible on top.

The priority of highlights is determined by the order in which they are defined in the CSS. Highlights defined later in the stylesheet have higher priority and will be rendered on top of earlier highlights. This is analogous to the stacking order of elements with different z-index values.

Example: Basic Highlight Priority

Consider the following CSS:

            
::selection {
 background-color: lightblue;
 color: black;
}

::highlight(custom-highlight) {
 background-color: lightcoral;
 color: white;
}

            

              
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In this case, if both ::selection and ::highlight(custom-highlight) apply to the same text range, the ::highlight(custom-highlight) will take precedence because it’s defined later in the stylesheet.

Example: Registering a Custom Highlight

Before using ::highlight, you typically need to register the custom property in JavaScript. Here's a simplified example:

            
if (CSS.registerProperty) {
 CSS.registerProperty({
 name: '--custom-highlight-color',
 syntax: '',
 inherits: false,
 initialValue: 'yellow',
 });
}

            

              
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And the corresponding CSS:

            
::highlight(my-custom-highlight) {
 background-color: var(--custom-highlight-color);
}

            

              
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Practical Use Cases for Custom Highlight Priority

Let's explore some practical use cases where controlling highlight priority can significantly improve the user experience:

1. Search Result Highlighting

When displaying search results, you often want to highlight the search terms within the content. If the user also selects text containing the search term, you might want the search highlight to remain visible underneath the selection highlight, or vice versa, depending on the desired effect.

Scenario: A user searches for "global accessibility" on a webpage. The search results are highlighted in yellow. The user then selects a portion of the text that includes "global accessibility".

Implementation:

            
.search-highlight {
 background-color: yellow;
}

::selection {
 background-color: lightblue;
 color: black;
}

            

              
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By defining ::selection after .search-highlight, the selection highlight will be on top. You could reverse the order to keep the search term always highlighted even when selected.

2. Syntax Highlighting in Code Editors

Code editors often use syntax highlighting to improve readability. When a user selects a block of code, you might want the syntax highlighting to remain visible underneath the selection highlight to preserve the code structure.

Scenario: A user selects a block of Python code in an online code editor. The code editor uses syntax highlighting to differentiate keywords, variables, and comments.

Implementation:

            
.keyword {
 color: blue;
}

.comment {
 color: gray;
}

::selection {
 background-color: rgba(0, 0, 255, 0.1);
}

            

              
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In this case, the syntax highlighting styles (.keyword, .comment) will be applied first, and the ::selection highlight will be rendered on top, providing a subtle visual cue without obscuring the syntax highlighting.

3. Collaboration and Annotations

In collaborative documents or annotation tools, different users might highlight different sections of the text. Controlling highlight priority can help differentiate between different users' highlights and maintain a clear visual hierarchy.

Scenario: Three users (Alice, Bob, and Charlie) are collaborating on a document. Alice highlights text in green, Bob highlights text in yellow, and Charlie highlights text in red.

Implementation:

            
.alice-highlight {
 background-color: green;
}

.bob-highlight {
 background-color: yellow;
}

.charlie-highlight {
 background-color: red;
}

::selection {
 background-color: rgba(0, 0, 255, 0.1);
}

            

              
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The ::selection highlight will be rendered on top of the user-specific highlights, allowing users to select text without completely obscuring the existing annotations.

4. Error Highlighting in Forms

When validating forms, it's important to clearly indicate which fields contain errors. Custom highlights can be used to visually emphasize the error fields. Controlling the highlight priority ensures that the error highlight remains visible even when the user selects the erroneous field.

Scenario: A user submits a form with an invalid email address. The email field is highlighted in red to indicate the error.

Implementation:

            
.error-highlight {
 background-color: red;
 color: white;
}

::selection {
 background-color: rgba(0, 0, 255, 0.1);
}

            

              
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The .error-highlight will be applied to the erroneous field, and the ::selection highlight will be rendered on top, allowing the user to select the field while still being aware of the error.

Accessibility Considerations

When customizing text highlights, it’s crucial to consider accessibility. Ensure that the highlight colors provide sufficient contrast with the text color to meet WCAG (Web Content Accessibility Guidelines) standards. Also, provide alternative visual cues for users who may have difficulty perceiving color.

1. Color Contrast

Use a color contrast checker to ensure that the contrast ratio between the highlight background color and the text color meets the minimum requirements specified in WCAG. A contrast ratio of at least 4.5:1 is recommended for normal text and 3:1 for large text.

2. Alternative Visual Cues

Provide alternative visual cues in addition to color to indicate highlighted text. This could include using a different font weight, adding an underline, or using a border.

3. Keyboard Accessibility

Ensure that the custom highlights are also applied when the user navigates through the text using the keyboard. Use the :focus pseudo-class to style the focused element and provide a clear visual indication of which element is currently selected.

4. Screen Reader Compatibility

Test your custom highlights with screen readers to ensure that the highlighted text is properly announced to users with visual impairments. Use ARIA attributes to provide additional context and information about the highlighted text.

Internationalization (i18n) Considerations

Text selection and highlighting can behave differently across different languages and scripts. Consider the following internationalization aspects when implementing custom highlights:

1. Text Direction (RTL/LTR)

Ensure that the highlight direction is consistent with the text direction. In right-to-left (RTL) languages, the highlight should start from the right and extend to the left.

2. Character Sets

Test your custom highlights with different character sets to ensure that they are displayed correctly. Some character sets may require specific font settings or encoding to render properly.

3. Word Boundaries

Be aware that word boundaries can vary across different languages. Ensure that the highlight is applied to the entire word, even if it contains characters that are not considered word characters in English.

4. Language-Specific Styling

You may need to apply different highlight styles based on the language of the content. Use the :lang() pseudo-class to target specific languages and apply language-specific styling.

Example: Highlighting text in Arabic (RTL):

            
:lang(ar) {
 direction: rtl;
}

::selection {
 background-color: lightblue;
 color: black;
}

            

              
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Advanced Techniques and Future Directions

1. CSS Painting API

The CSS Painting API allows you to create highly customized highlights using JavaScript to define the painting logic. This opens up a wide range of possibilities, such as creating animated highlights, adding complex visual effects, or integrating with external data sources.

2. Custom Highlight Painters

You can create custom highlight painters that extend the functionality of the CSS Painting API. This allows you to encapsulate reusable highlighting logic and apply it to different elements or highlight regions.

3. Integrating with JavaScript Frameworks

JavaScript frameworks like React, Angular, and Vue.js can be used to manage custom highlights dynamically. This allows you to create interactive highlighting tools that respond to user input or data changes.

Browser Compatibility

The CSS Custom Highlight API is still relatively new, and browser compatibility may vary. Check the latest browser compatibility tables on websites like Can I use... to ensure that the API is supported in your target browsers. Consider using polyfills or alternative approaches for older browsers that do not support the API.

Conclusion

The CSS Custom Highlight API provides a powerful way to control text selection layer priority and manage highlights for global accessibility. By understanding the key concepts and techniques discussed in this blog post, you can create visually appealing, accessible, and internationalized web experiences that enhance the user experience for everyone. Remember to always consider accessibility, internationalization, and browser compatibility when implementing custom highlights.

By carefully managing highlight priority and considering the needs of a global audience, you can create web experiences that are both visually appealing and highly accessible, ensuring that everyone can enjoy the content you create. The future of CSS highlights is bright, with the CSS Painting API and custom highlight painters paving the way for even more innovative and creative highlighting techniques.