MDN HTML
MDN's reference page on HTLM (HyperText Markup Language) will give you all the tools you need to get started with HTML and creates an easy page to bookmark and use as a reference to all things HTML. This page provides access to tutorials, articles, and references to every element and attribute.
The tutorials included will range from beginner levels perfect for learning to more advance topics to really expand your skills.
Learn more about HTML
MDN CSS
MDN continues to give you the resources you need to continue on your learning journey and offering a set of reference for those refresher moments we all need form time to time. MDN's CSS (Cascading Style Sheet) reference page offers tutorials in ranging skill levels. Their CSS reference section is aimed towards seasoned Web developers and describes every property and concept of CSS.
This page also offers some great tools that allow you to validate your CSS and also gives you developer tools to view and edit your page in a live environment.
Learn more about CSS
MDN Summary
MDN's website is a great resource for those just starting out and a great reference tool for those experienced. MDN is a trusted resource because it is open sourced and worked on as a collaborative documents. it is unbiased and has even received contributions from such partners including Microsoft and the W3C. Contributions are reviewed and then merged with existing content.
Learn more about MDN