CSS Custom Highlight Intersection: Mastering Multiple Selection Overlap

The ::highlight pseudo-element in CSS offers powerful control over the styling of text selections. While basic text highlighting is straightforward, managing the intersection of multiple selections and customizing their overlapping areas requires a deeper understanding. This article delves into advanced techniques for mastering CSS custom highlight intersection, providing practical examples and actionable insights for creating polished and accessible user interfaces.

Understanding the Basics: The ::highlight Pseudo-element

The ::highlight pseudo-element allows you to style text that has been selected by the user. This capability extends beyond simple background colors and text colors; it enables developers to apply a wide range of CSS properties to selected text, including borders, shadows, and even animations. This is a significant advancement over relying solely on the browser's default highlighting, which is often inconsistent and doesn't always align with a website's design.

Basic Syntax

The basic syntax for using ::highlight involves targeting specific Selection objects via CSS selectors or through the CSS Custom Highlight API.

Here's a simple example:

            ::highlight {
  background-color: yellow;
  color: black;
}

            

              
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This code snippet will change the background color of any selected text to yellow and the text color to black. This is a global style that applies to all selections on the page. To target specific selections, you need to use the CSS Custom Highlight API.

The CSS Custom Highlight API: Fine-grained Control

The CSS Custom Highlight API provides a more granular way to manage and style text selections. It allows you to create named highlights and apply specific styles to them. This is particularly useful when you need to differentiate between different types of selections or when dealing with overlapping selections.

Creating and Applying Named Highlights

To use the CSS Custom Highlight API, you'll need to use JavaScript to create and apply named highlights. Here's a step-by-step guide:

1. Register a Named Highlight: Use CSS.registerProperty() to register a custom property that will be used to style your highlight. This is generally done once at the start of your script.
2. Create a Range: Define the start and end points of the text you want to highlight using Range objects.
3. Get the Selection Object: Obtain the current selection using window.getSelection().
4. Add the Range to the Selection: Use the addRange() method to add the range to the selection. This is often paired with first removing all existing ranges using removeAllRanges().
5. Apply Styles: Define the style for the highlight. This may involve setting CSS variables.

Here's an example demonstrating this process:

            // Register the highlight property (run once)
CSS.registerProperty({
  name: '--my-highlight-color',
  syntax: '',
  inherits: false,
  initialValue: 'rgba(255, 165, 0, 0.5)' // Semi-transparent orange
});

// Function to apply the highlight
function applyHighlight(startNode, startOffset, endNode, endOffset, highlightName) {
  // 1. Create a Range object
  const range = document.createRange();
  range.setStart(startNode, startOffset);
  range.setEnd(endNode, endOffset);

  // 2. Get the Selection object
  const selection = window.getSelection();

  // 3. Clear existing selections and add the new range
  selection.removeAllRanges();
  selection.addRange(range);

  // 4. Apply the highlight using ::highlight(highlightName) in CSS
  //   No JavaScript needed for direct styling, CSS handles the look
}

// Example usage:
const myElement = document.getElementById('my-text-element');
// Assuming myElement contains the text you want to highlight
applyHighlight(myElement.firstChild, 5, myElement.firstChild, 15, 'my-highlight');

            

              
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And the corresponding CSS:

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

            

              
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In this example, we're registering the --my-highlight-color custom property, then highlighting the text from character 5 to 15 within the `my-text-element`. The CSS then uses the registered property to set the background color.

International Considerations for Text Ranges

When working with text ranges in different languages, it's crucial to consider Unicode and character encoding. The startOffset and endOffset values represent character indices, which can be problematic with languages that use multi-byte characters or grapheme clusters. For example, in some East Asian languages, a single character might be represented by multiple bytes. You might need to use libraries that handle Unicode correctly to ensure that your highlights are applied accurately across different languages.

Additionally, text direction (left-to-right vs. right-to-left) can also affect how ranges are calculated and applied. Be sure to test your highlighting implementation with various languages and scripts to ensure proper functionality and rendering.

Handling Multiple Selection Overlap

The real challenge arises when dealing with multiple selections that overlap. The default browser behavior for overlapping selections can be unpredictable, and often doesn't provide the desired visual effect. The CSS Custom Highlight API provides tools to manage this overlap, allowing you to control how different highlights interact with each other.

Understanding the Problem

When multiple selections overlap, the browser typically applies the styles of the last selection made. This can lead to visual inconsistencies and a confusing user experience. For example, if you have two overlapping highlights, one green and one blue, the overlapping area might only show the blue highlight, completely hiding the green one. To address this, you need to implement a strategy for resolving the overlap.

Strategies for Resolving Overlap

There are several strategies you can use to resolve overlapping highlights:

• Prioritization: Assign different priorities to different types of highlights. The highlight with the highest priority will take precedence in the overlapping area.
• Blending: Blend the colors of the overlapping highlights to create a new color. This can provide a visually appealing way to indicate the presence of multiple selections.
• Layering: Use CSS properties like z-index to control the stacking order of the highlights. This allows you to create a layered effect, where one highlight appears on top of another.
• Custom Rendering: For complex scenarios, you can use JavaScript to analyze the overlapping ranges and render custom visual elements, such as borders or icons, to indicate the presence of multiple selections.

Implementing Prioritization

Prioritization involves assigning a priority level to each highlight and ensuring that the highlight with the highest priority is displayed in the overlapping area. This can be achieved by carefully managing the order in which highlights are applied and using CSS to control their appearance.

Here's an example of how you might implement prioritization using CSS variables and JavaScript:

            // Define highlight priorities
const highlightPriorities = {
  'important-highlight': 2,
  'secondary-highlight': 1,
  'default-highlight': 0
};

// Function to apply a highlight with a specific priority
function applyHighlightWithPriority(startNode, startOffset, endNode, endOffset, highlightName) {
  // (Same range creation and selection logic as before)
  const range = document.createRange();
  range.setStart(startNode, startOffset);
  range.setEnd(endNode, endOffset);
  const selection = window.getSelection();
  selection.removeAllRanges();
  selection.addRange(range);

  // Apply the highlight name to a custom property on the selection.
  // No direct CSS manipulation here; relies on the CSS below.
  //  This requires polyfilling for browsers lacking CSS.supports.
  //  Also, use caution with the security implications of setting
  //  arbitrary styles via JavaScript and make sure only trusted code can do this.
  let priority = highlightPriorities[highlightName] || 0;

  if (CSS.supports('selector(::highlight(' + highlightName + ''))')) {
      // No need to directly manipulate the style object if CSS.supports is available
      // The CSS will handle the rest based on the selection.
  }
  else {
      // Fallback behavior, apply styles directly.  This is NOT recommended
      //  for production code, as it's difficult to manage and maintain.
      //   It is better to use the CSS Custom Highlight API with a polyfill, or simply
      //   gracefully degrade the highlighting feature if the browser doesn't support it.
      console.warn("CSS.supports not supported, direct style manipulation may be needed");
  }

}

            

              
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And the corresponding CSS:

            ::highlight(default-highlight) {
  background-color: lightgray;
  z-index: 0; // Lowest priority
}

::highlight(secondary-highlight) {
  background-color: lightblue;
  z-index: 1;
}

::highlight(important-highlight) {
  background-color: orange;
  z-index: 2; // Highest priority
}

/* Use to address z-index issues in some browsers*/
::highlight {
  position: relative;
}

            

              
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In this example, each highlight type is assigned a z-index value, with higher values indicating higher priority. The position: relative declaration on the ::highlight pseudo-element might be needed to ensure that z-index works correctly in some browsers.

Implementing Blending

Blending involves combining the colors of overlapping highlights to create a new color. This can be achieved using CSS blend modes or by manipulating the background colors of the highlights using JavaScript.

Here's an example of how you might implement blending using CSS blend modes:

            ::highlight(highlight-1) {
  background-color: rgba(255, 0, 0, 0.5); /* Red with 50% opacity */
  mix-blend-mode: multiply;
}

::highlight(highlight-2) {
  background-color: rgba(0, 0, 255, 0.5); /* Blue with 50% opacity */
  mix-blend-mode: multiply;
}

            

              
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In this example, the mix-blend-mode: multiply property is used to blend the colors of the overlapping highlights. When a red highlight overlaps a blue highlight, the resulting color in the overlapping area will be purple.

Implementing Layering

Layering involves using CSS properties like z-index to control the stacking order of the highlights. This allows you to create a layered effect, where one highlight appears on top of another. This approach is similar to prioritization, but it provides more flexibility in terms of visual styling.

The example provided in the Prioritization section already demonstrates how to implement layering using z-index.

Implementing Custom Rendering

For complex scenarios, you can use JavaScript to analyze the overlapping ranges and render custom visual elements, such as borders or icons, to indicate the presence of multiple selections. This approach provides the most flexibility but also requires the most effort to implement.

Here's a high-level overview of how you might implement custom rendering:

1. Analyze Overlapping Ranges: Use JavaScript to iterate through the selections and identify any overlapping ranges.
2. Create Custom Elements: Create HTML elements, such as <span> or <div>, to represent the custom visual elements.
3. Position Elements: Position the custom elements precisely over the overlapping ranges using JavaScript and CSS.
4. Style Elements: Apply custom styles to the elements to create the desired visual effect.
5. Insert Elements: Insert the custom elements into the DOM, ensuring that they are positioned correctly relative to the text.

This approach can be complex and requires careful attention to detail, but it provides the ultimate control over the appearance of overlapping highlights. It is important to handle edge cases, such as text wrapping and changes in font size, to ensure that the custom elements remain correctly positioned.

Accessibility Considerations

When customizing text highlights, it's crucial to consider accessibility to ensure that your website is usable by people with disabilities. Here are some important considerations:

• Color Contrast: Ensure that the color contrast between the highlight color and the text color is sufficient for people with low vision. Use a color contrast checker to verify that the contrast ratio meets accessibility guidelines. WebAIM's contrast checker is a great resource.
• Keyboard Navigation: Ensure that users can navigate and interact with highlighted text using the keyboard. This may involve adding ARIA attributes to the highlighted elements to provide semantic information to assistive technologies.
• Screen Reader Compatibility: Test your highlighting implementation with screen readers to ensure that the highlighted text is properly announced. Provide descriptive text for the highlights to help users understand their purpose and meaning.
• Avoid Relying Solely on Color: Don't rely solely on color to convey meaning. Provide alternative visual cues, such as borders or icons, to ensure that the highlights are accessible to people who are colorblind.

Real-World Examples and Use Cases

Custom highlight intersection can be used in a variety of real-world scenarios to enhance the user experience. Here are some examples:

• Code Editors: Code editors can use custom highlights to indicate syntax errors, warnings, and other important information. Overlapping highlights can be used to show multiple issues in the same area of code.
• Document Editors: Document editors can use custom highlights to indicate tracked changes, comments, and annotations. Overlapping highlights can be used to show multiple revisions in the same section of text.
• Search Results: Search results pages can use custom highlights to show the search terms within the search results. Overlapping highlights can be used to show multiple search terms that appear in the same area of text.
• Annotation Tools: Annotation tools can use custom highlights to allow users to highlight and annotate text. Overlapping highlights can be used to show different types of annotations in the same area of text.

Best Practices

Here are some best practices to follow when implementing CSS custom highlight intersection:

• Use the CSS Custom Highlight API: The CSS Custom Highlight API provides the most flexible and efficient way to manage and style text selections.
• Consider Accessibility: Always consider accessibility when customizing text highlights. Ensure that the highlights are usable by people with disabilities.
• Test Thoroughly: Test your highlighting implementation thoroughly across different browsers, devices, and languages to ensure that it works correctly.
• Use a Consistent Visual Style: Use a consistent visual style for your highlights to provide a clear and intuitive user experience.
• Document Your Code: Document your code clearly and concisely to make it easier to maintain and update.

Conclusion

CSS custom highlight intersection is a powerful technique that allows you to create visually appealing and informative user interfaces. By mastering the ::highlight pseudo-element and the CSS Custom Highlight API, you can control the appearance of text selections and manage the intersection of multiple selections to provide a seamless and accessible user experience. Remember to prioritize accessibility, test thoroughly, and use a consistent visual style to ensure that your highlighting implementation is effective and user-friendly.

Further Resources

• MDN Web Docs: ::highlight
• MDN Web Docs: CSS Custom Highlight API
• WebAIM Color Contrast Checker