MDN HTML
This page contains information about HTML. It includes references resources and tutorials. This is a good place to find help with HTML code and to learn more about it.
MDN CSS
Like the HTML page this page also has resources and information, but about CSS. There are tutorials as well. This is a place to look for help with CSS.
Summary
It is always better to avoid untrusted sources when looking for code help. Code from these places may be fine, but you never know. sometimes there can be malicious code hiding in code coming from untrusted sources. This is why you should stick with trusted sources when seeking help with code.