That is what the HTML 4 !doctype Tag - https://www.quackit.com/html/html_4/tags/doctype_html_public... - was meant for. Ideally, an old client that cannot parse a newer HTML spec should inform the user to update the client. Developers could then either support old versions of their site (more easily, in such a case) or can simply decide not to deal with outdated browsers. (In fact, isn't that what most BiGTech sites actually do now, through convoluted browser feature detections - "Your browser is no longer supported ...").