HTML and CSS

HTML, CSS, JS Documentation and Tutorials

MDN is no doubt the greatest resource for web developers. It contains documentation (and examples) for HTML elements and attributes, CSS properties, and Javascript APIs and syntax. It even has a ton of great tutorials on various topics. You should really, really, really use MDN.

W3Schools has some pretty decent tutorials introducing HTML, CSS, and JS, and also has some reference for HTML elements, Javascript syntax, etc (similar but less comprehensive than MDN).

Mobile Design (Flexbox, Grid, Media Queries)

Unlike laptops and PCs, phones are very small (citation needed), and often in portrait, not landscape mode. So, special design and CSS considerations are needed to make sure websites still work and look nice on phones.

Media queries are a good way to do this - media queries can apply CSS depending on the screen size.

Grid and flexboxes are also cool. Go search them up.