> but why does the website that's sending me HTML and CSS and Javascript need to know?
The website doesn't get told by the browser, but the website is sending you Javascript, and the browser will tell the Javascript when the Javascript politely inquires as to the width and height of the root html element, or some element with text in a funny font in it, and the Javascript is then free to report home.
I think hiding layout information like that from Javascript isn't really within reach without a radically different model that breaks a ton of websites.
And when I block js, most websites are still readable. (some even look better!) The fact that some sites work just fine without js mean that most could. Certainly the sites load much, much quickly without the js. 90% of the time all I want is the text, which loads perfectly without it. JS is a huge waste of resources with no real benefit to consumers. Some web apps need it, yes, but even those could still do fine without it. (until recently there was still an HTML gmail app which worked just fine.)
The website doesn't get told by the browser, but the website is sending you Javascript, and the browser will tell the Javascript when the Javascript politely inquires as to the width and height of the root html element, or some element with text in a funny font in it, and the Javascript is then free to report home.
I think hiding layout information like that from Javascript isn't really within reach without a radically different model that breaks a ton of websites.