I actually prefer limited content width for prose content. Full width content on wide screens requires moving eyes all the way from one side to another for every line.
If you could easily shrink a tab, I would prefer websites to not limit text width. Since you can't, I sorta prefer them to do it, though it's much worse than the user controlling it in a nice per tab way
(2) user stylesheets (permanent solution, but you could have multiple and use an extension to enable disable different widths)
(3) responsive mode (in dev tools, most flexible, but most cumbersome to reach)
(4) Other extensions
There are easy ways to resize the viewport, so the premise is false.
It should also be almost trivial to create a browser extension for this, if it doesn't even exist yet.
This is beautified if somebody wants to see how it is done.
function() {
var myBody = document.getElementsByTagName('body')[0];
var myBodyWidth = myBody.style.width;
if (!myBodyWidth || myBodyWidth === 'auto' || myBodyWidth === 'inherit') {
myBody.style.width = '1200px';
myBody.style.marginLeft = 'auto';
myBody.style.marginRight = 'auto';
myBody.style.position = 'relative';
myBody.style.cssFloat = 'none';
} else {
myBody.style.width = 'auto';
myBody.style.position = 'static';
}
}