They're so simple that it's hard to believe they just work. I'm almost surprised that browser vendors haven't pushed the functionality out of existence just because, in favor of ever more complicated solutions.

Similarly, while prototyping an internal company wiki I made bookmarklets to fix some Sharepoint shenanigans and add features that otherwise would've required mucking about with extensions (tough when half the company was forced to use IE) or even worse, Sharepoint apps. The entire IT staff had a major WTF moment after explaining just how simple the whole thing was.