You might try DaisyDiff, a Java library that diffs (compares) HTML files. It highlights added and removed words and annotates changes to the styling.
Either use commandline (see DaisyDiff examples) or java api. See Main Class and DaisyDiff Class to get you going.
Here is a blogpost (in german) with some snippet generating a html report.
EDIT after comment
To simply check if there are any differences and catch those files, you don't need any addons,
use something like :
<project>
<fileset dir="/path/to/folder1" includes="**/*.html" id="diff">
<different targetdir="/path/to/folder2" ignoreFileTimes="true"/>
</fileset>
<pathconvert refid="diff" pathsep="${line.separator}" property="htmldiff"/>
<!-- if fileset diff is empty, ${htmldiff} and htmldiff.txt are also empty -->
<!-- echo to file, one file per line -->
<echo file="htmldiff.txt">${htmldiff}</echo>
</project>