What Is XHTML?
XHTML stands for EXtensible HyperText Markup Language
XHTML is almost identical to HTML
XHTML is stricter than HTML
XHTML is HTML defined as an XML application
XHTML is supported by all major browsers
Many pages on the internet contain "bad" HTML.
This HTML code works fine in most browsers (even if it does not follow the HTML rules):
<title>This is bad HTML</title>
<p>This is a paragraph
来源：www.w3school.com.cn 和 www.w3schools.com