The HTML parser has always (or for a long time) allowed elements and attributes to have a wide variety of valid characters and names, but the javascript DOM APIs to create the same elements and attributes are more strict and don't match the parser. This change relaxes the validation of the javascript DOM APIs to match the HTML parser. More context here: https://github.com/whatwg/dom/issues/849 I don't anticipate any compat issues from this change because all of the previously allowed element/attribute names are still allowed with the new behavior.