GOV.UK’s guidance on developing using progressive enhancement is pretty great in all departments. It begins with this solid advice:
you should start by making your page work with just HTML, before adding anything else like Cascading Style Sheets (CSS) and JavaScript. This is because HTML is the most resilient layer. If the HTML fails there’s no web page. Should the CSS or JavaScript fail, the HTML will still render correctly.
I particularly like the section where they address the misconception that a resilient baseline is only required in places where the user has explicitly disabled JavaScript and therefore not worth worrying about.
You should not assume the reason for designing a service that works without CSS or JavaScript is because a user chooses to switch these off. There are many situations when extra layers can fail to load or are filtered.
As their subsequent list of scenarios illustrates, a user turning JavaScript off is probably the least likely of a range of reasons why extra layers on top of HTML can fail.
Relatedly, I’ve often found that Everyone has JavaScript, right? serves as a great go-to reference for these sorts of conversations around resilience.