You could argue the browser is NOT showing your code the way you intended

I don't think that's a good example because how line breaks are treated is defined in the HTML standard.

A better example might be "reader" mode on mobile browsers? Or that mode in Opera where their servers act as a proxy to compress images and reduce the amount of data required to load the website for slow connections.

Actually the default HTML form and button elements are good one. Chromium and Firefox have different default styles.