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.
MDN DOM
This page explaind what the Document Object Model, DOM is and how it works. It has resources and references like the others. This is a place to go to learn about the DOM and how to work with it.
MDN JavaScript
This page has links to information on HTML and CSS as well as information, tutorials, and resources for Javascript. This is a place to find out about how javascript works and help with code.
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.