Well, the situation you describe, is probably best handled by CSS media queries.

In case you’re not familiar, you can write:

@media (width < 800px) {
  h1 {
    font-size: 110%;
  }
}

…to get smaller headings on mobile, for example.

But yeah, reality may be more ugly. Especially, if you’re using a bulky JS framework, it may be easier to do the JS dance.