Add support for floating point boundaries

The linting is currently failing on `master`, but as far as I can tell, this patch does not introduce any new errors.

Commit message:

> According to the W3C's documentation of the `range` input type [1], the
> `min`, `max`, `step`, and `value` attributes may all be specified as
> floating point numbers.
>
> [1] http://www.w3.org/TR/html-markup/input.range.html