Simply adding "
encoding="utf-8"" to the .html file does not work. You'll need to actually convert the .html file to Unicode.
There seem to be several different Tamil code pages in use; you'll have to find out the encoding of your .html file and then use a converter to convert it to Unicode.
Simply google for Tamil Uncode converter and Tamil Unicode fonts and pick one that works.
Alternatively, save your source file as Unicode with your word processor/editor.
You could also try to copy the text to the clipboard, paste it into
BabelPad and then save it as a Unicode text file.
BTW, even if you manage to convert your source file to Unicode there's no guarantee that this will solve your problem.
I wouldn't bet on ADE, because the current version is pretty limited when it comes to non-Latin alphabets. However, there's a good chance that a properly encoded Tamil ePub with Unicode .html source files might work at least on the iPad.