Introduction to the difference between HTML and XHTML, and HTML4 and HTML5 tags

1. XHTML elements must be correctly nested

2. XHTML elements must be closed, and empty tags must also be closed. If<br/>

3. XHTML elements must be lowercase

4. XHTML document must have a root element

5. The XHTML attribute name must be lowercase, the attribute value must be quoted, and the attribute cannot be abbreviated. For example: <input checked=checked />

6. XHTML uses the id attribute instead of the name attribute.

Note: An extra space should be added before the / symbol to make your XHTML compatible with today's browsers

7. Language attribute (lang)lang attribute is applied to almost all XHTML elements. It defines the type of language used for the content inside an element. If you use the lang attribute in an element, you must add an extra xml:lang, like this: <div lang=no xml:lang=no>Heia Norge!</div>

Three XML document types for XHTML 1.0

XHTML 1.0 specifies three XML document types to correspond to the above three DTDs.