I recently dealt with a client whose high-traffic store was constantly throwing layout errors. The culprit? A “clever” snippet of code meant to inject custom attributes into product images using preg_replace. It worked fine until someone uploaded an image with an attribute order the regex didn’t expect. Total mess. The site’s layout imploded because of a single misplaced double-quote.
My first instinct, years ago, would have been to just “fix” the regex. Maybe add a few more lookarounds. But that’s just digging a deeper hole. Trust me on this: if you’re still using regular expressions to parse HTML in 2025, you’re building on sand. With the WordPress HTML API improvements in 6.9, there’s officially no excuse left for brittle string manipulation.
The Power of a Public serialize_token
The headline for most of us in the trenches is that WP_HTML_Processor::serialize_token() is now public. Previously, the processor was great at finding things, but actually rebuilding the HTML safely was… tricky. Now, you can iterate through a document and spit out perfectly normalized HTML for every token you encounter. This is massive for security and reliability.
Here’s how I handled that client’s image attribute problem using the updated API. Instead of guessing with regex, we now let WordPress handle the heavy lifting of normalization.
function bbioon_safe_image_processor( $html ) {
$processor = WP_HTML_Processor::create_fragment( $html );
$output = '';
while ( $processor->next_token() ) {
// If it's an image, let's add our tracking attribute.
if ( 'IMG' === $processor->get_tag() ) {
$processor->set_attribute( 'data-bbioon-track', 'product-view' );
}
// This is the magic. It returns the well-formed representation of the token.
$output .= $processor->serialize_token();
}
return $output;
}This approach, which builds on the concepts discussed in the official dev notes, ensures that even if the input HTML is “junk,” the output is well-formed. No more broken tags.
Mapping the JavaScript Dataset Gap
If you’ve ever worked with the Interactivity API or custom blocks, you know that the mapping between HTML attributes like data-wp-bind--class and the JavaScript .dataset.wpBind-class is a headache. It’s not intuitive. 6.9 introduces wp_js_dataset_name() and wp_html_custom_data_attribute_name(). Simple, but essential. Period.
It solves that annoying “how many dashes do I need?” guessing game. It’s the kind of quiet improvement that saves an hour of debugging on a Friday afternoon. And we’ve all been there.
Semantic Testing with assertEqualHTML
Finally, let’s talk about testing. If you’re writing unit tests for your plugins (and you should be), you’ve likely dealt with assertSame() failing because one string used single quotes and the other used double. It’s a false positive that wastes time. The new assertEqualHTML() method in WP_UnitTestClass compares the meaning of the HTML, not just the characters. It knows that <img src="a.jpg" loading="lazy"> is the same as <img loading='lazy' src='a.jpg' />.
Look, this stuff gets complicated fast. If you’re tired of debugging someone else’s mess and just want your site to work, drop my team a line. We’ve probably seen it before. The HTML API is becoming a powerhouse, but you need to know how to wield it without overcomplicating your codebase.
The lesson here is simple: stop fighting HTML with strings. Use the tools Core is giving you. It’ll save your sanity and your client’s budget in the long run.
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