Actually, the viewport meta tag is (being made) CSS [1], and it’s way beyond me why unstyled HTML is rendered unreadably small on mobile devices without it (and without any fixed layout to speak of)...
> it’s way beyond me why unstyled HTML is rendered unreadably small on mobile devices without it
Because browsers that did that would be both breaking spec compliance and introducing vendor-specific rendering, two things that IE proved were very very bad.
Every browser in use today has a default stylesheet that injects styles into every site (if not explicitly overriden). Even for sites that have their own stylesheets.
It's just that a) they're really ugly, and b) the mobile ones are ironically not mobile optimized.
They’re not necessarily overridden. Percentages and ems are units that are derived relative to their inherited style. If you make your browser default have a base font of 24px instead of 14px and someone back in 2010 made their site css defined as
html {
font-size: 2em;
}
Then your site’s 28px default font size that worked just fine on every browser now becomes 48px on mobile and breaks everything.
The problem arises without the viewport meta tag, and not on all browsers or devices. This site does use this tag, which has become sort of mandatory even when there’s no layout.
[1] https://www.w3.org/TR/css-device-adapt-1/