The customizable select's selectedcontent mirroring, replayed after parsing.

This is not a rule of the tree construction algorithm. The forms chapter of the standard runs "update a select's selectedcontent" from the DOM insertion steps as the parser inserts nodes, so the tree a browser ends up with shows the clone. This module produces that tree once the parse is complete, with the forms chapter's definitions as fetched on 2026-09-15:

• a select's enabled selectedcontent is its first selectedcontent descendant in tree order, unless the select has multiple or that selectedcontent is disabled (it sits under a nested select, an option, or another selectedcontent);
• the list of options is the option descendants of the select in tree order, not descending into select, hr, option, datalist, or a nested optgroup;
• selectedness starts from the selected attribute, then the selectedness setting algorithm: without multiple, all but the last selected option lose selectedness, and with display size 1 and nothing selected the first non-disabled option is selected;
• the first selected option's children are cloned into the selectedcontent, or it is cleared when none is selected.

See: https://html.spec.whatwg.org/multipage/form-elements.html#update-a-select's-selectedcontent