"HTML began as a very simple language for creating web pages. XHTML arose when web development expanded and professional programmers needed something beyond HTML 4. The strict version of XHTML (eXtensible HyperText Markup Language) was considered much more precise and predictable than HTML 4."
XHtml was never very successful.
Html 5 is the next version of Html with features from both Html 4 and Xhtml.
Read more
here[
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