Do not flag render-blocking CSS as an issue if certain criteria are met

This site for example has a single CSS resource of 7.4kb (transfer size). https://www.hopstreet.be/

- just a `link rel="stylesheet"` tag in the html head without any conditions in a `media` attribute
- same fat CSS bundle for all breakpoints
- same fat CSS bundle for all pages, templates, ...
- depending on the breakpoint and template only 10% - 20% of this resource will be used

In practice this site would not become meaningfully faster by splitting the CSS in any way.
And it would become slower if we deferred it with javascript.

- it is a very small file
- it is the last external resource in the html head (after async script resources)
- second page performance is also important

Would it make sense to have some thresholds/rules were this warning/issue isn't triggered?