a11y-debugging

Core Concepts

Accessibility Tree vs DOM: Visually hiding an element (e.g.,

CSS opacity: 0

) behaves differently for screen readers than

display: none

aria-hidden="true"

. The

take_snapshot

tool returns the accessibility tree of the page, which represents what assistive technologies "see", making it the most reliable source of truth for semantic structure.

Reading web.dev documentation: If you need to research specific accessibility guidelines (like

https://web.dev/articles/accessible-tap-targets

), you can append

.md.txt

to the URL (e.g.,

https://web.dev/articles/accessible-tap-targets.md.txt

) to fetch the clean, raw markdown version. This is much easier to read!

Workflow Patterns

1. Browser Issues & Audits

Chrome automatically checks for common accessibility problems. Use

list_console_messages

to check for these native audits first:

```
types
```
:
```
["issue"]
```
```
includePreservedMessages
```
:
```
true
```
(to catch issues that occurred during page load)

This often reveals missing labels, invalid ARIA attributes, and other critical errors without manual investigation.

2. Semantics & Structure

The accessibility tree exposes the heading hierarchy and semantic landmarks.

Navigate to the page.
Use
```
take_snapshot
```
to capture the accessibility tree.
Check Heading Levels: Ensure heading levels (
```
h1
```
,
```
h2
```
,
```
h3
```
, etc.) are logical and do not skip levels. The snapshot will include heading roles.
Content Reordering: Verify that the DOM order (which drives the accessibility tree) matches the visual reading order. Use
```
take_screenshot
```
to inspect the visual layout and compare it against the snapshot structure to catch CSS floats or absolute positioning that jumbles the logical flow.

3. Labels, Forms & Text Alternatives

Locate buttons, inputs, and images in the
```
take_snapshot
```
output.
Ensure interactive elements have an accessible name (e.g., a button should not just say
```
""
```
if it only contains an icon).

Orphaned Inputs: Verify that all form inputs have associated labels. Use

evaluate_script

to check for inputs missing

id

(for

label[for]

) or

aria-label

() =>
  Array.from(document.querySelectorAll('input, select, textarea'))
    .filter(i => {
      const hasId = i.id && document.querySelector(`label[for="${i.id}"]`);
      const hasAria =
        i.getAttribute('aria-label') || i.getAttribute('aria-labelledby');
      return !hasId && !hasAria && !i.closest('label');
    })
    .map(i => ({
      tag: i.tagName,
      id: i.id,
      name: i.name,
      placeholder: i.placeholder,
    }));


4.  Check images for `alt` text.

### 4. Focus & Keyboard Navigation

Testing "keyboard traps" and proper focus management without visual feedback relies on tracking the focused element.

1.  Use the `press_key` tool with `"Tab"` or `"Shift+Tab"` to move focus.
2.  Use `take_snapshot` to capture the updated accessibility tree.
3.  Locate the element marked as focused in the snapshot to verify focus moved to the expected interactive element.
4.  If a modal opens, focus must move into the modal and "trap" within it until closed.

### 5. Tap Targets and Visuals

According to web.dev, tap targets should be at least 48x48 pixels with sufficient spacing. Since the accessibility tree doesn't show sizes, use `evaluate_script`:

```js
// Usage in console: copy, paste, and call with element: fn(element)
el => {
 const rect = el.getBoundingClientRect();
 return {width: rect.width, height: rect.height};
};

Pass the element's
uid
from the snapshot as an argument to
evaluate_script
.

6. Color Contrast

To verify color contrast ratios, start by checking for native accessibility issues:

Call
```
list_console_messages
```
with
```
types: ["issue"]
```
.
Look for "Low Contrast" issues in the output.

If native audits do not report issues (which may happen in some headless environments) or if you need to check a specific element manually, you can use the following script as a fallback approximation.

Note: This script uses a simplified algorithm and may not account for transparency, gradients, or background images. For production-grade auditing, consider injecting

axe-core

el => {
  function getRGB(colorStr) {
    const match = colorStr.match(/rgba?\((\d+),\s*(\d+),\s*(\d+)/);
    return match
      ? [parseInt(match[1]), parseInt(match[2]), parseInt(match[3])]
      : [255, 255, 255];
  }
  function luminance(r, g, b) {
    const a = [r, g, b].map(function (v) {
      v /= 255;
      return v <= 0.03928 ? v / 12.92 : Math.pow((v + 0.055) / 1.055, 2.4);
    });
    return a[0] * 0.2126 + a[1] * 0.7152 + a[2] * 0.0722;
  }

  const style = window.getComputedStyle(el);
  const fg = getRGB(style.color);
  let bg = getRGB(style.backgroundColor);

  // Basic contrast calculation (Note: Doesn't account for transparency over background images)
  const l1 = luminance(fg[0], fg[1], fg[2]);
  const l2 = luminance(bg[0], bg[1], bg[2]);
  const ratio = (Math.max(l1, l2) + 0.05) / (Math.min(l1, l2) + 0.05);

  return {
    color: style.color,
    bg: style.backgroundColor,
    contrastRatio: ratio.toFixed(2),
  };
};

Pass the element's
uid
to test the contrast against WCAG AA (4.5:1 for normal text, 3:1 for large text).

7. Global Page Checks

Verify document-level accessibility settings often missed in component testing:

() => ({
  lang:
    document.documentElement.lang ||
    'MISSING - Screen readers need this for pronunciation',
  title: document.title || 'MISSING - Required for context',
  viewport:
    document.querySelector('meta[name="viewport"]')?.content ||
    'MISSING - Check for user-scalable=no (bad practice)',
  reducedMotion: window.matchMedia('(prefers-reduced-motion: reduce)').matches
    ? 'Enabled'
    : 'Disabled',
});

Troubleshooting

If standard a11y queries fail or the

evaluate_script

snippets return unexpected results:

Visual Inspection: If automated scripts cannot determine contrast (e.g., text over gradient images or complex backgrounds), use
```
take_screenshot
```
to capture the element. While models cannot measure exact contrast ratios from images, they can visually assess legibility and identifying obvious issues.