The past is not dead! Actually, it’s not even past. You know who said that? Faulkner. And he was right.
No, this is neither “midnight in Paris” (the film), nor an attempt to paraphrase William Faulkner’s “Requiem for a Nun”. Extensible Hyper Text Markup Language (XHTML) is a (now superseded) W3C recommendation whose goal is to improve the HTMl language by reimplementing it on top of a stricter ruleset directly derived from XML. The idea behind this specification is that any XHTML web page can be treated as an XML document as well, with all benefits arising from that decision.
There was a time in the past when it was strongly recommended, if not enforced by laws an regulations to create accessible web pages using XHTML. Nowadays, the XHTML syntax is part of the HTML Living Standard.