We're not checking the validity of HTML for the demo page. This was noticed in #119.

It would be nice to have something added to check for HTML validity - either something that lints the HTML files, or something that makes the build task fail when HTML isn't valid.

I would be looking for recommendations on how to do this and would be open to any pull request that accomplishes the goal!

When start clicking number buttons everything is ok, but if I start number with 0 this zero remains, while it should be overriten by the next non-zero number :)