Trac ticket: Core-63694.

This patch introduces a new CSS helper module containing a new function, wp_split_class_names(). This function wraps some code to rely on the HTML API to take an HTML class attribute value and return a Generator to iterate over the classes in that value.

Design review requests

• The name isn’t great. It iterates over the class names, splits them, but also “deduplicates” them according to the parsing rules for a class attribute no-quirks-mode HTML and it decodes HTML entities. It also MUST represent a full class attribute because the parsing of trailing character references which are missing a semicolon or otherwise incomplete is dependent on whether they fall at the end of a string.
  • wp_classname_walker()
  • wp_walk_class_attribute()
  • wp_unique_classnames()
• Should it be more useful to people wanting to conditionally add class names? Something more akin to classnames() in JS? We could pass varargs which are string|false or an array of additional class names to add.

function wp_classnames( 'wp-block-paragraph', array( 'display-wide' => $is_wide ) ) { … };

Background

Many existing functions perform ad-hoc parsing of CSS class names, usually by splitting on a space character. However, there are issues with this approach:

• There is no decoding of HTML character references, which is normative inside HTML attributes.
• There is no handling of null bytes.
• Class names can be split by more than just the space character.
• There is no handling of duplicates, and while mostly benign, code forgetting to account for duplicates can lead to defects.

The new function handles the nuances to let developers focus on reading CSS class names, adding new class names, and removing class names. This serves a middleground between legacy code interacting with CSS class names in isolation and code processing full HTML documents.