I mean, a build step is not required to build a website, but I'd say anyone who wants to have more than one HTML page and one stylesheet will probably want some sort of build step sooner rather than later.

Like, if you have two or more pages, you probably want them to share the same header or footer, and unless you want to A) repeat the same markup on each page or B) inject them with a client-side script, you will need some sort of build step. There are more accessible solutions out there, like Hugo.

How else are you going to achieve that? Sure you could use PHP, but I don't see how that is more accessible or maintainable than having a build step.