It's so easy to misuse animations in UI design. A great post on tasteful, effective animation: https://uxdesign.cc/good-to-great-ui-animation-tips-7850805c12e5
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The "line of death", where the browser UI splits between trusted UI elements and UI controlled by the website.
Also argues that HTTP warnings are better than HTTPS padlocks, because there's incentive to spoof padlocks lower on the page.
https://emilymstark.com/2022/12/18/death-to-the-line-of-death.html
I'm a huge fan of Swift's 'Error Handling Rationale' design document: https://github.com/swiftlang/swift/blob/9315673c003875158852579bd1f33480cdec5461/docs/ErrorHandlingRationale.md#fundamentals
It carefully defines terminology and compares with other languages, so you can understand Swift's position and preference in the design space.
You can often estimate the age of a website based on how well it displays on mobile.
All my sites end up with media queries in the CSS for narrow screens. It's so hard to design a single UI that scales from phone to desktop otherwise.