Html code completion uses the standard Vim code completion mechanism like so:
<ht<Ctrl-X><Ctrl-U>
<html>
<he<Ctrl-X><Ctrl-U>
<html>
<head>
<lin<Ctrl-X><Ctrl-U>
<html>
<head>
<link ty<Ctrl-X><Ctrl-U>
<html>
<head>
<link type
...
Warning
Html completion has been disabled on Windows when using the headless version of eclim because of a native call which blocks indefinitely. Hopefully in the future this issue will be resolved or a work around found.