Ignoring my previous issue, I ended up stumbling across an issue when modifying the value of attributes that were already attached to an element, as well as modifying existing text content.

The previous behavior for setAttribute was to simply append a new attribute to the attrs array, while this does work for new elements, it causes issues when attempting to override existing attributes. When parse5 serializes the AST, it only recognizes the first attribute of a type, and ignores any others with the same name. This behavior is un-intuitive as calling setAttribute in some situations may have no effect on the serialized content.

This issue is easily fixed by simply removing any old attributes with the given name.

I also stumbled across a very similar issue with setTextContent when modifying the text content of some elements. This issue was also caused by appending new text nodes without removing any previous content beforehand. While the old behavior does act similar to the document.write() function, the name setTextContent doesn't match the functionality. If this behavior is intended, I would recommend renaming setTextContent to appendTextContent and creating a new function for setTextContent which behaves intuitively.

This issue was also easily fixed by removing any text nodes from a parent element before appending the new content.