MDN HTML
This page defines HTML. It provides references, resources, and tutorials. There are articles to read about advanced topics that surround HTML. This is a great source for help with HTML code and to learn more about it.
MDN CSS
Like the HTML page this page defines CSS and provides resources and information about it. It contains tutorials and reference materials as well as a CSS Cookbook with code that can be used as a starting point for designing a page. It is an equally great source for information and help with CSS.
Summary
It is always best to turn to a trusted source when looking for information on the web. Code from other places may be fine, but you never know if it contains sneaky bad code. Sometimes there can be malicious code hiding unseen when code comes from untrusted sources. This is why you should stick with reliable and reputable sources when seeking help with code.