I also thought about this part of OCD: the only thing I noticed (although this is far from a comprehensive solution) is that the floating-point elements demonstrate the behavior that you are executing. This script shows a live version (screenshot below) in which the two are divsidentical, except that there is an ad float: left;that prevents text from being highlighted from the entire page.
If the problem is big enough for you (although I would say otherwise), you could put your main container to ensure that text selection is limited. @solendil's “solution” seems like a massive overkill, presenting a huge lack of usability (as annoying as not being able to select text) for the sake of slight visual annoyance.

: @AndreZS position, static, ( , , ). -, WebKit, , , : , - , .