about:legacy-compat made its way into HTML5 as the system identifier/URL of a (fictive) external SGML declaration set to contain declarations for the elements a

about:legacy-compat made its way into HTML5 as the system identifier/URL of a (fictive) external SGML declaration set to contain declarations for the elements and attributes of HTML. It's required to make an HTML5 document a valid SGML application; if it weren't there, the declaration set would be treated as containing no declarations at all, which doesn't work for eg. empty (HTML "void") elements such as img and br, and also for writing enumerated attributes in their short forms such as in <option selected> etc.