But when using a proportional font, as most of us do use when typing into a browser
Again, not to nitpick, but as a web developer, I just want to clarify one thing. Usually, when you're composing content for the web, you don't specify the font. You're just specifying the content. The font is up to the individual viewer's browser. They could specify whatever font they want for their "default" font, including a monospaced font like Courier New.
Now, again, of course you
can force a particular font, size, colour, whatever on your content, so that you can be certain all your viewers will see your page in purple 12-point Verdana or whatever. But you're not
supposed to, because visitors might have perfectly valid reasons for wanting the page to render using the font they've specified themselves, such as poor eyesight (they want text to appear in 18-point), colour blindness (they can't read purple text on a pink background), or maybe they're visually impaired and using a text-to-speech reader that doesn't recognize whatever font you've specified.
"Best Practices" of the web dictate that you should just provide the content, and let the end users' browsers handle the specific formatting and rendering. But thanks to CSS, people have gotten it into their head that the Web is just one big Microsoft Word document, and they want the page to look exactly the same in everybody's browser, on every platform, regardless of peoples' individual needs and preferences.
All that to say, unless you're violating best practices and explicitely specifying a typeface, then you can't be certain that your visitors are reading your text in a proportional-spaced font. They could be viewing it in a monospaced font.