Is lang=unknown attribute valid?
问题 Given an HTML document in a specific language (english). I have defined a lang attribute on the tag : <html lang="en"> Some texts in the page are written in another language (for example french) : <span lang="fr"> blabla... </span> But, if I can not identify the language, but I know it is NOT english, can I set "unknown" as a valid value for lang attribute? <span lang="unknown"> blabla... </span> I read this in w3c documentation, but I am not sure if "the default value is [...] unknown" means