vertpush's resources: a catch-all list of organised references, from website-building to art.
HTML and CSS
w3schools' tutorials: total game-changers, I always have w3schools open in another tab as a guide.
sadgrl's layout builder: a really useful tool for html/css beginners! generates a basic layout for neocities with guidance throughout the code to allow customisation as a starting point.
RV's free script codes: an assortment of page, text and mouse scripts (using JavaScript), such as creating a rainbow trail, making rolling or bouncing text, having a clock attached to the mouse, etc.