• HTML versus XHTML - What's different?

All elements and attributes must be lowercase because XHTML is case sensitive
example- <br /> and not <BR>

All elements must be terminated, including empty elements like br and img
note- non-empty elements are closed by their end tag
example of non-empty element format- <b>make it bold</b>
note- but empty elements are closed by  />
example-1 of empty element format- <BR> becomes <br />
example-2 of empty element format- <IMG... > becomes <img... />

Element order is preserved, meaning they must close in correct order
example- wrong   <b><i>make it bold and italic</b></i>
example- correct <b><i>make it bold and italic</i></b>

Special characters must be represented by their character identities
example- so Boys & Girls Club must be Boys &amp; Girls Club
NOTE: a semi-colon MUST close the character identity

All attribute values must be in quotation marks
example1- <td rowspan="3"> and not <td rowspan=3>
example2- <img src="pic.gif" /> and not <img src=pic.gif />

Link to W3C for more on XHTML vs HTML