It really depends on what you're trying to test. You can hit the backend to prove stability of those elements, but you can't hit the backend to show what the user will experience while using your website.
Most of the raw HTML generation, rendering is done by frameworks and browsers respectively and they're pretty standard / efficient pieces of software.
I guess we all come at this with our own assumptions. When I think about testing, I'm also thinking about the base experience, not just adding resources and adding load.