This will be website style sheet dependent. If the website's style sheets impose a maximum width, the width will be constrained. If not, they will likely flow

This will be website style sheet dependent. If the website's style sheets impose a maximum width, the width will be constrained. If not, they will likely flow to the edges. But, do note that it is possible to run your browser with a window width less than the width of the monitor (i.e. not maximized) if "flow to the edges" is also too wide. My browser windows are all about 55% of the width of my 16:10 monitor, and I seldom ever maximize the browser window. And when I do maximize it, most often for map websites, when I'm done I return the window to its normal about 55% width.