Actually, my advice is the opposite. Put your templates in a template directory, and then keep the .html suffix. Then, when I load the file in most html editors, they recognise it immediately from the suffix. They don't quite "get" the tmpl_ tags, but I can highlight things and mark them "strong" or "em"phasis or "div" or whatever. (For some reason, I don't think my editor allows blink. Oh well :-})

This way, I get all the shortcuts for adding tags, I get the syntax highlighting, and I know they're templates from the directory they're in.

I actually started with your advice: new name, outside directory. I went back on the name because I found I was always telling my editor to pretend that .tmpl was actually HTML, so I cheated by putting them in files that the editor understood were HTML.

And I'm even more glad I did now - I have another templating system based on Text::ScriptTemplate that I use at work where .tmpl is now taken for that - and having my editor autoload .tmpl as HTML would make those look very, very silly. (No, that's not used for webpages, but for almost anything else - including shell scripts.)