To comment code in the WordPress Editor, you must comment code within an element and not around it.
Examples
To comment out the following anchor…
<a href="http://google.com" title="Google">Google</a>
do the following:
<!--a href="http://google.com" title="Google">Google</a-->
Here’s another example, starting with the h3 tag and ending with the image tag:
<!--h3>SOME NEIGHBORHOOD NOTABLES
Named after Edmond S. Meany, the history professor, registrar and prominent civic leader for whom the UW's Meany Hall is also named, the Edmond Meany Hotel (1930-1931) remains an impressive landmark in the University District and is currently home to the Hotel Deca.
<img src="http://www.washington.edu/community/map/drumhellerfountain1910.jpg/image_sidebar" alt="Drumheller Fountain" width="150" height="89" /-->
