Even if one were limited to a single window, I’d hazard to guess that most modern web frameworks and languages have some degree of templating and pre-processing capabilities, which let you have logically contained files with HTML and CSS that decompose/render into separate files.

I’ve made setups like that on a number of projects (ASP.Net & various .Net web frameworks). keeping clean separation of concerns, proper cascading, but also a simplified development experience where the ‘component’ file contained all relevant code, markup, and local styling.