Building web pages with XHTML 1.1

In XHTML, all attribute values must be enclosed in either single quotes ('…') or double quotes ("…"). HTML allowed you to use unquoted attributes.

Furthermore, XHTML requires that all attributes must have a value. HTML accepted attribute names with no value.

<title border=1>…

<title border='1'>…

<input type=checkbox checked>…

<input type="checkbox" checked="checked">…