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Salma Alam-Naylor
Salma Alam-Naylor

Posted on • Updated on • Originally published at

How to make your code blocks accessible on your website

If you’re a developer or technical writer who publishes content on the internet, you’ll want to make sure your code examples are presented beautifully for your audience to consume. 

The good news is, there are plenty of tools available to adorn your website with fancy-looking code blocks in your blog posts, that mimic current and trendy IDE themes. For example, uses the Okaidia theme from Prism.js to present code blocks.

The bad news is, most prebuilt themes (especially those with darker colour palettes) come loaded with accessibility issues such as failing colour contrast checks, meaning your audience may have difficulties consuming your code examples. Additionally, your website will be penalised in web search results for failing accessibility checks.

This means we need to do a little more work in CSS to make our code blocks accessible. Let’s take a look.

How to check accessibility on your website

If you’re developing in Google Chrome or Microsoft Edge, you can access the Google Lighthouse tools from your developer console.

Quick tip — some Chrome plugins can affect the way Lighthouse runs in your browser — so it’s always recommended to run these checks in an incognito window without any plugins activated.

A screenshot showing the Google Lighthouse dev tools tab with the Accessibility checkbox checked

You can use Google Lighthouse to run a variety of reports on your web pages. Check the 'Accessibility' checkbox and click ‘Generate report’.

A screenshot showing a 100% Accessibility score on the Google Lighthouse dev tools tab

Here’s an accessibility check for this blog post (including code blocks) which produces a satisfying score of 100%! 🎉 But remember — as the report shows — there are always areas of your website that automated tools might not be able to assess correctly — so you should always carry out manual checks as well.

axe — Web Accessibility Testing

I’ve been using this Chrome plugin for a number of years, and it has helped me incredibly in improving my knowledge of accessibility.

Install the plugin, open your Chrome dev tools, and navigate to the ‘axe’ tab.

A screenshot of the axe accessibility panel in Chrome dev tools with options to scan all of my page and scan part of my page

What’s great about axe is that it shows the HTML code for any issues that it has detected. For the same blog post we scanned with Lighthouse above, axe finds 11 issues.

That's not so good! But let’s investigate.

A screenshot showing an axe accessibility tool issue description of failing colour contrast in a YouTube iframe embed

Axe is describing colour contrast issues inside a YouTube iframe embed. It’s not related to any CSS that's bundled with the web page, and it’s not ideal. We might be able to fix it if we can target the iframe CSS effectively, or use a different thumbnail on the YouTube video. But for now, it's good to know!

WAVE Evaluation Tool

The third tool that is integral to my accessibility workflow is the Wave Evaluation Tool Chrome extension. This tool is great for highlighting semantic HTML and ensuring you’ve included the necessary aria labels.

Here’s an example of Wave in action.

A screenshot showing a Wave Evaluation Tool summary tab overlaid on the home page of

Click on ‘View details’ to see a detailed breakdown of the summary.

A screenshot showing a Wave Evaluation Tool details tab overlaid on the home page of

Fixing code block accessibility issues

There are two main issues to look for in your code blocks, both of which axe picked up for me.

Ensure the contrast between foreground and background colors meets WCAG 2 AA contrast ratio thresholds

The theme I chose to use from Prism.js is beautiful, but some of the colours failed the colour contrast checks.

Here is a code block that failed a colour contrast check:

A screenshot of a code block that fails colour contrast

This is described in axe alongside the underlying HTML:

A screenshot of the axe accessibility panel in Chrome dev tools showing a failing colour contrast check

Finding accessible colours

Another neat tool I use is the Colour Contrast checker that’s also available as a Chrome extension. Activate the extension, and use the eyedropper to select background and foreground colours to view whether it passes Web Content Accessibility Guidelines (WCAG).

A screenshot of the Colour Contrast Checker Chrome extension showing a failing colour contrast check

The beautiful thing about this tool is you can move the hue, saturation and light sliders to find colours that pass accessibility guidelines without having to leave your browser page. Using the slider tools, I was able to find colours that passed colour contrast checks, and added the following overrides to my global CSS file:

 /* accessibility fixes for prismjs */

  .token.comment {
    color: #adb8c2 !important;

  .token.important {
    color: #f3a344 !important;

  .token.constant {
    color: #fc92b6 !important;
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Elements that have scrollable content should be accessible by keyboard

This accessibility violation was a little trickier to solve, and is the primary reason for writing this blog post.

A screenshot showing an axe accessibility tool issue description of scrollable content failure

Prism.js ships with the following code that caused this violation, which causes overflow text to scroll rather than wrap:

pre[class*="language-"] {
   padding: 1em;
   overflow: auto;
   border-radius: 0.3em;
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Code blocks are not focusable by a keyboard (whereas anchor tags are focusable, for example). This means that if your audience is using only a keyboard to navigate the site, they will be unable to access the content that has overflowed the container, which would usually be scrolled with a mouse.

The way to fix this accessibility issue is to ensure the text will wrap when necessary inside the <code> tag.

Here’s the code I used to prevent the scrolling behaviour and make the text wrap onto a new line:

code[class*="language-"] {
  white-space: pre-wrap;
  word-break: break-all;
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In conclusion

Prebuilt IDE-like code block themes like the ones available from Prism.js are beautiful, but if you want to use one, ensure you check for accessibility issues such as colour contrast and scrolling behaviour. This will ensure your audience can use your site effectively, and your site ranks well in search engines for accessibility.

And remember: Build stuff, learn things, love what you do.

Top comments (5)

grahamthedevrel profile image

Great article but do you have evidence that accessibility has any bearing on search rankings as you mention it a couple of times and from experience I can say that search rankings do not appear to affected by accessibility (which is a shame as I spend all my days making sites accessible).

Would love to see some evidence of it if you have it as it adds another reason to implement accessibility and makes my job even easier of getting buy in from project leads, business owners etc.

rpearce profile image
Robert Pearce

While there is a good deal of overlap between SEO and a11y (, it looks like as of last year Google apparently has no plans:

IMO, all web developers should be striving to create accessible experiences as part of their daily work, but there are still significant knowledge and tooling gaps that need to be filled across the industry.

Getting buy-in to go back and fix things, though, is hard until the company gets sued or misses out on a big client. Then it's easy!

grahamthedevrel profile image

Oh I have a bank of arguments that make getting accessibility on the table relatively easy but saying that Google was actively promoting for would be the icing on the cake!

Unfortunately as you pointed out this is not the case, maybe it is the next thing after the User Experience update in May (one can only hope 🤣).

whitep4nth3r profile image
Salma Alam-Naylor

Thanks for your comment! You raise some good questions.

I don't know where you are in the world, but there are laws in support of accessibility, for example in the EU, the European Accessibility Act was introduced in 2019 - This might be really useful in helping get buy in from other areas of the profession.

With this article I was taking a broad accessibility approach and prioritising humans with regards to colour contrast and non-keyboard focusable elements. However, I've found quite a few articles that discuss search engines being able to more effectively crawl and index web pages that use best accessibility practises. For example, if your web page includes an audio file/movie file with no transcript or textual content, a search engine won't be able to 'understand it' and therefore won't be able to index it and/or rank it according to search terms. This might not be as relevant for optimising text code blocks for people who don't use a mouse, but however as we know, accessibility is a hugely broad issue.

What's interesting is I found this article from 2005 that feels like it was written much more recently - - because the nuances of it are still so relevant today - and maybe more so.

Thank you for spending all your days making sites accessible :)

grahamthedevrel profile image

Yeah I am aware of indirect improvements in SEO, just wondered if you had seen some case study where Google actively pushed items up its rankings because it was accessible, sadly it seems we are still not there yet.

Nice article you linked though, who knows maybe the next Google algorithm update after the "User Experience Update" coming in May will be the "Inclusion Update" and they will penalise sites for accessibility issues 😋