CSS Comment breaks validation

## What's the issue?

When a string containing disallowed class prefix is present in a CSS comment it is flagged as a validation error.

## How do we reproduce the issue?

Add a CSS comment within `` containing a disallowed class name, such as:

``` <style amp-custom> /* Can't use .i-amphtml- selectors here */ ```

## What browsers are affected?

All browsers

## Which AMP version is affected?

Probably not a new issue, first identified on a page using AMP version 2103122145004

Reproduce the issue by placing a disallowed class prefix in a comment inside , then trace the CSS validation entry point that reports the error. The fix is complete when such comments are ignored while actual disallowed selectors remain flagged, with the relevant validation checks passing.

Written by the indexing model from the issue text.