Things I would like every young web engineer to learn:

• anything you can do in CSS + HTML, you should do in CSS + HTML
• framework du jour is not a platform, it's a high-interest loan against your future capacity. The platform is the platform
• understanding the memory hierarchy always matters
• client-side isn't easier than the server, and "generalists" usually suck at client-side. Mind the (packet) gap
• managers who are not technical are not useful
• put users first, always

Second-order things to learn:

• the way browsers work isn't static, but it also isn't changing that fast. Learn as much as you can and update every few years; particularly about networking and the rendering loop.
• JS is the slowest way to do anything on the web. Never let it become the way you do everything.
• a11y isn't nice-to-have, it's the job
• shipping fast almost never matters as much as quality, & there are simple heuristics you can use to understand the difference