If you opt in, a simple page like this ends up looking pretty modern anyway, but plain, while if you opt out, your site is often rendered as if made for an older desktop monitor, like this one.

Another way of putting the problem, we preserved old desktop styles rather than invent new default responsive styles for touch-based browsers.

Without a standard like grey backgrounds and blue links to fall back on, websites had to start making decisions, especially when sites go responsive. At that point it is easier to invent new styles… there aren’t that many useful defaults to fall back on.

This is particularly true for HTML5 form controls, which rarely look good in any browser without styling…

But perhaps there’s hope for the future as new design language becomes HTML tags. E.g. details has an arrow, datalist builds a native autocomplete, and so on. It’s a shame that dialog doesn’t have native formatting tho, or that the idea of creating new, modern default styles hasn’t caught on yet in a cross-browser way.