7 http://www.w3.org/TR/html401/types.html#h-6.1
413 Differences Between HTML and XHTML
Since all attribute names and values are case sensitive in XHTML, selectors are
always case sensitive.
The simplest way to mitigate any potential issues surrounding case sensitivity is
to always use lowercase for everything in your markup and CSS, where possible.
If that??™s not possible, make sure the case you use is consistent between your CSS
and your document markup.
Optional Tags
The HTML specification allows us to omit some tags. Several end tags are optional,
for instance
and
, but they don??™t matter from a CSS point of view. What is
important to consider is that in HTML 4, the start tags are optional in four instances:
, , , and . The corresponding elements will exist in the
document object model (DOM) tree whether or not the tags are present in the markup.
It??™s now considered best practice to include these tags explicitly, but an HTML
document can be valid without them.
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