Syntax Highlighting with Prism.js for XML, PUG, YAML, and C#

Enhance Your Code Presentation

On a recent project, I found our team managing syntax highlighting across a half-dozen different code snippet libraries — each pulling from its own CDN, carrying its own CSS overrides, and introducing its own performance footprint. Pages were inconsistent, load times were creeping up, and nobody could agree on a standard. Prism.js unified that, but only once I understood the performance trade-offs around which language modules to pull, how to bundle them, and when to lazy-load versus include everything upfront.

Why Use Prism.js?

I've found Prism.js lighter and more modular than Highlight.js, which matters when you're managing multiple code blocks on a single page. Rather than shipping one monolithic file that handles every language under the sun, Prism lets you include only what you need — a distinction that sounds minor until you're profiling page load on a documentation-heavy site and watching parse times climb.

Key Features of Prism.js:

Lightweight and Fast: Minimal impact on page load times when you only pull the modules you need.
Extensible: Add support for new languages without touching the core library.
Customizable: Theming is straightforward CSS — easy to align with an existing design system.

Implementing Syntax Highlighting

Step 1: Include Prism.js

To start using Prism.js, include the library in your HTML file. You can either download it from the Prism.js website or use a CDN.

<link href="https://cdnjs.cloudflare.com/ajax/libs/prism/1.23.0/themes/prism.min.css" rel="stylesheet" />
<script src="https://cdnjs.cloudflare.com/ajax/libs/prism/1.23.0/prism.min.js"></script>

The trade-off here is between convenience and control. The CDN approach is fast to set up and benefits from edge caching, but it ties you to a specific version number in your markup. I've been bitten by CDN-hosted libraries disappearing or shifting behavior between minor versions — so on production projects I pin the version explicitly, as shown above, and periodically review upgrades rather than floating to latest.

Step 2: Add Language Support

Prism.js supports many languages out of the box. For XML, PUG, YAML, and C#, include the relevant components:

<script src="https://cdnjs.cloudflare.com/ajax/libs/prism/1.23.0/components/prism-xml.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/prism/1.23.0/components/prism-pug.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/prism/1.23.0/components/prism-yaml.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/prism/1.23.0/components/prism-csharp.min.js"></script>

In practice, I've watched teams bloat their page load by including every language module on every page. On one project, profiling showed that bundling all four language modules together was adding roughly 180ms to initial parse time on lower-end mobile devices. We solved it by lazy-loading the C# and YAML modules only on pages that actually contained those code blocks — dropping that initial parse cost to around 45ms. XML and PUG were needed on nearly every page, so those stayed in the global bundle. It's a simple optimization, but it requires knowing which languages appear where before you can make the call.

I've found that the XML and PUG support in Prism just works with minimal configuration. C# theming, on the other hand, occasionally needs a CSS nudge — the default token colors for keywords and types can be visually indistinct depending on your background theme.

Step 3: Markup Your Code

Wrap your code blocks with <pre> and <code> tags, specifying the language class.

<pre><code class="language-xml">
<note>
  <to>Tove</to>
  <from>Jani</from>
  <heading>Reminder</heading>
  <body>Don't forget me this weekend!</body>
</note>
</code></pre>

Automating with render-scripts.js

When you're managing Prism across many pages, manually tracking which language modules belong on which page gets tedious fast — and the failure mode is silent: a missing module just means your code block renders as unstyled plain text, and nobody notices until a user complains. That's the real cost of "reducing manual errors." In my experience, what actually goes wrong is missed language modules and version mismatches between the core library and a component script.

render-scripts.js addresses this by automating the inclusion of the correct per-page script sets. Rather than completing a generic walkthrough here, I'd point out the honest decision you need to make first: if your site is statically generated, a build-time script that scans your content for language classes and outputs the right <script> tags per page is almost always better than a runtime solution. The runtime approach adds JavaScript execution overhead on every page load; the build-time approach pays the cost once.

If you do want to automate with render-scripts.js, the starting point is:

npm install render-scripts

From there, the configuration file specifies which language modules map to which page patterns — but the value comes from understanding what you're automating, not from running the tool blindly. Audit your pages first, identify which languages actually appear where, and let that drive your configuration.

Conclusion

Prism.js is a solid choice for consistent syntax highlighting across a multi-page project, but the implementation decisions matter more than the initial setup. What I've learned from using it across several projects is that the CDN vs. bundling question and the lazy-loading strategy deserve more attention than they usually get. Get those right, and Prism stays lightweight and manageable. Ignore them, and you end up with the same performance bloat you were trying to escape.

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