What's a web ui hack that all of us should know about?

Right-click, inspect element. Rewrite text, change background colors and font colors to make it more legible, remove overlays and ad popups, find image URLs buried beneath gallery left/right controls (twitter, for example). Fix typos or remove uncomfortable content before making screenshots. Make content scrollable again when a website tries to paywall you. Rewrite content to trick your friends. That's pretty much it.