They don't have to use compiled, minified and obfuscated css, vectorized graphics, and user-agent detected custom fonts
but if you don't, then you are unoptimized for other realities
websites in the past used system fonts, you can still do that.
websites in the past used low quality raster images, or high quality raster images. those websites don't look good at higher resolutions. conditional logic to fix those sites is messy.
you can still make sites like that.