The frosted glass effect was a huge eye-opener for me the first I saw it in the early 2000s. At the time I thought PNG-based translucency was the be-all-end-all of GUI design and nothing would ever trump it. I genuinely believed there was no UI effect you could not create by simply adding an alpha channel and allowing for translucency and layers overlaying on top of each other. Drop-shadows, glass effects, all of it can be fully done by PNGs that overlay on top of each other. Then I saw a frosted glass / background effect and I was awestruck, here was a UI effect that wasn't simply overlaying pixels over pixels.... I'll never forget that moment

spoiler: and so, I left frontend :D