Make sure all your tags have a closing tag or have a slash at the end 
(<br ></br > can be denoted <br />). Problems should have no warnings! 
Plain HTML pages _may_ be able to handles some of the warnings, but problems 
should be warning free.

An example:

<p > Paragraph text </p >
<hr > replaced with <hr />
<i ><b > text </b ></i > is correct (<i ><b > text </i ></b > is wrong)