Customizable select is a feature update to the select element which contains several parts listed here: https://github.com/whatwg/html/issues/9799 - Relax <select> parser https://bugs.webkit.org/show_bug.cgi?id=283732 - appearance:base for <select> or appearance:base-select - https://drafts.csswg.org/css-ui-4/#base-appearance - https://github.com/whatwg/html/pull/10629 - ::picker(select) pseudo-element https://drafts.csswg.org/css-forms-1/#selectordef-picker - ::picker-icon pseudo-element https://drafts.csswg.org/css-forms-1/#picker-icon-pseudo-element - ::checkmark pseudo-element https://drafts.csswg.org/css-forms-1/#checkmark-element - <selectedcontent> element https://github.com/whatwg/html/pull/10633 - UA stylesheet for base appearance <select> https://github.com/whatwg/html/pull/10670