XHTML is a stricter version of HTML that is XML-compliant. It has the following key differences from HTML:
1. XHTML documents require a DOCTYPE declaration and xmlns attribute. Elements like <html>, <head>, <title>, and <body> are also mandatory.
2. All XHTML elements must be properly nested, closed, and in lowercase.
3. Attribute names must be in lowercase and values must be quoted. Attribute minimization is forbidden.
4. Following these XHTML rules ensures documents can be processed and rendered accurately across browsers now and in the future.