How to Use Proxies for Web Accessibility Testing

Understanding the Role of Proxies in Web Accessibility Testing

Like the art of kintsugi, which highlights rather than hides imperfections, web accessibility testing seeks to reveal barriers so that digital experiences can be made whole for everyone. Proxies, acting as intermediaries, allow testers to perceive the web through different lenses—geographic, technological, and perceptual—mirroring the Buddhist concept of “shoshin” (初心), or beginner’s mind: open, curious, and free from assumptions.

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Types of Proxies Used in Accessibility Testing

Proxy Type	Primary Use Case	Advantages	Disadvantages
HTTP/S Proxy	Inspect and modify web traffic, test alternate content delivery	Flexible, supports SSL decryption	Configuration can be complex
SOCKS Proxy	Route all traffic at the TCP level	Protocol-agnostic, works with many applications	Limited content inspection
Reverse Proxy	Simulate server-side changes, test accessibility at source	Centralized control, caching	Not suitable for client-side scripts
Accessibility Proxy	Specialized for simulating assistive tech (e.g., screen readers)	Focused on a11y scenarios	Fewer tools available

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Why Use Proxies for Accessibility Testing?

Just as the Zen garden reveals the landscape through careful arrangement, proxies reveal hidden pathways and obstacles in digital accessibility, such as:

• Testing alternative text delivery to screen readers
• Simulating slow or unreliable connections (mimicking real user constraints)
• Intercepting and modifying requests to assess response to assistive technologies
• Accessing geo-restricted content to test localization and language support

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Setting Up an HTTP Proxy for Accessibility Testing

Example: Using Fiddler

1. Install Fiddler: Download and install Fiddler from the official site.
2. Configure Browser/Device: Set your browser’s proxy to 127.0.0.1:8888 (the default Fiddler port).
3. Capture Traffic: Open Fiddler and start capturing. All HTTP/S requests will route through Fiddler.
4. Modify Requests: Use Fiddler’s “AutoResponder” to swap images, CSS, or scripts with alternatives that simulate assistive tech behaviors.
5. Test Output: Open your site in the browser, using screen readers like NVDA or JAWS to verify how changes impact accessibility.

Code Example: FiddlerScript to Swap Images with ALT Text

import System;
import Fiddler;

class Handlers
{
    public static function OnBeforeResponse(oSession: Session) {
        if (oSession.uriContains("example.com/image.jpg")) {
            oSession.utilReplaceInResponse("<img src='image.jpg'", "<img src='image.jpg' alt='Descriptive ALT text'");
        }
    }
}

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Simulating Assistive Technology with Accessibility Proxies

Like a calligrapher testing many brushes, an accessibility proxy can help you observe how different assistive technologies interpret your site.

• axe-proxy: Integrates Axe-core accessibility engine into a proxy server. It intercepts HTTP requests, injects Axe into responses, and returns automated accessibility reports.
• Set up axe-proxy:
  1. Install Node.js.
  2. npm install -g axe-proxy
  3. Run: axe-proxy --port 8080
  4. Set browser/system proxy to localhost:8080
  5. Browse your site; review accessibility violations reported by the proxy.

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Testing Slow Connections and Fallbacks

As a tea master slows the pour to appreciate aroma, simulate slow networks to test how your site responds. Users on assistive tech often have limited bandwidth.

• Using Charles Proxy:
  1. Open Charles.
  2. Go to Proxy > Throttle Settings.
  3. Enable throttling, choose speed (e.g., 56kbps).
  4. Observe site loading and fallback content, especially for images and scripts.

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Intercepting and Modifying HTTP Responses

To reflect on impermanence, test how your site handles missing, altered, or erroneous resources:

• With Fiddler or Charles, use rewrite tools to:
  • Remove or corrupt ARIA attributes
  • Block CSS to test focus visibility
  • Replace text with alternate language

Example: Removing ARIA Attributes with Mitmproxy

from mitmproxy import http

def response(flow: http.HTTPFlow) -> None:
    if b'aria-label' in flow.response.content:
        flow.response.content = flow.response.content.replace(b'aria-label', b'')

• Save as remove_aria.py
• Run: mitmproxy -s remove_aria.py

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Comparing Proxies for Accessibility Testing

Tool	Platform	Unique Features	Best Use Case	Link
Fiddler	Windows/Mac	AutoResponder, Inspectors	HTTP/S traffic manipulation	https://www.telerik.com/fiddler
Charles Proxy	Windows/Mac	Throttling, Map Local	Simulating slow networks, offline tests	https://www.charlesproxy.com/
mitmproxy	Cross-Platform	Python scripting, CLI	Custom response manipulation	https://mitmproxy.org/
axe-proxy	Node.js	Automated a11y analysis	Automated accessibility testing	https://github.com/dequelabs/axe-proxy

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Practical Use Cases and Step-by-Step Guidance

1. Testing Image ALT Fallbacks

   • Use Fiddler AutoResponder to remove images.
   • Test with screen readers to ensure descriptive ALT is announced.

2. Validating Keyboard Navigation

   • With Charles, block CSS to reveal tab order and focus styles.
   • Manually tab through the interface to ensure logical sequence.

3. Language and Localization

   • Use mitmproxy to replace text nodes with alternate languages.
   • Verify language attributes and announcements with screen readers.

4. Testing for ARIA Misuse

   • Intercept responses to remove or alter ARIA roles and properties.
   • Observe changes in assistive technology feedback.

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Cultural Insight: The Path of Continuous Refinement

In Japanese craftsmanship, “kaizen” signifies continuous improvement. Web accessibility, like the patient shaping of bonsai, is an ongoing process. Employing proxies, you adapt your site to the varied ways users experience the world, refining not just the technical structure, but the soul of the web itself.

For further reading, consult:
– WebAIM: Introduction to Web Accessibility
– W3C Accessibility Testing
– Deque University: Accessibility Testing Tools

Like the wise gardener, let your testing tools be varied, your methods deliberate, and your commitment to inclusion unwavering.