Storage and platform APIs

the value went out with every single request to that domain, whether it was needed or not.

Cookie

you saved the preference and it was still there a week later, on that browser only.

localStorage

the value survived a refresh and vanished when the tab was closed, and the other tab never saw it.

sessionStorage

you needed to keep megabytes of structured data on the device, so key-value strings were not enough.

IndexedDB

the cached data disappeared on its own because the device was low on space.

Storage quota

a script you are not running sits between the page and the network and answers requests itself.

Service worker

you stored whole request and response pairs so the service worker could answer them without the network.

Cache Storage

the app opened and worked on a train with no signal, then synced when the connection came back.

Offline-first

you stopped the polling when the tab went to the background and started it again when it came back.

Page visibility

you found out when the element scrolled into view without running any code on scroll.

IntersectionObserver

you reacted to the element changing size instead of listening for the window resizing.

ResizeObserver

you needed to know when someone else's script changed the DOM, so you watched the subtree.

MutationObserver

your page's stylesheet had no effect inside the widget because its DOM was in a separate tree.

Shadow DOM

you defined a tag with a dash in it and used it in HTML like any built-in element.

Custom element

you checked whether the API existed rather than checking which browser you were in.

Feature detection