I don't know what resources you've used to learn HTML/CSS but some people like Learn to Code HTML & CSS for the very basics. CSS has added a lot recently so Learn CSS should probably be the next stop (they recommend MDN's Introduction to HTML for HTML).
Essentially it's hard to remember things you don't use, so it's important to use them as soon as you learn them (and keep using them). It can also help to put together your own cheat sheets to remind your future self in your own words what you have learned (having to describe it will likely reinforce it or expose gaps in understanding that still need to be filled).
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For practising there are sites like Frontend Mentor that have free challenges; specifically Free, Newbie (level: 1), HTML & CSS;
Examples:
Order Summary Component; Newbie(1)
Space tourism multi-page website; Intermediate(3)
Other recommendations:
MDN has a fundamental layout comprehension assessment.
I don't know what resources you've used to learn HTML/CSS but some people like Learn to Code HTML & CSS for the very basics. CSS has added a lot recently so Learn CSS should probably be the next stop (they recommend MDN's Introduction to HTML for HTML).
Essentially it's hard to remember things you don't use, so it's important to use them as soon as you learn them (and keep using them). It can also help to put together your own cheat sheets to remind your future self in your own words what you have learned (having to describe it will likely reinforce it or expose gaps in understanding that still need to be filled).