MDN HTML
The HTML pages on the MDN break the language of HTML down into the essentials. There are sections about defining the terms and tutorials for beginners. In addition, there is a whole reference section with all the elements and attributes listed. I think the reference section will be most beneficial for me to use in the future since I am familiar with HTML and it probably has several elements and attributes that I haven't used yet.
MDN CSS
Like the HTML pages, there are several resources for CSS that include defining terms and tutorials. There is also a whole reference section with all the CSS properties. Again, I think the reference section will be most beneficial for me since I am familiar with CSS and I'm sure the reference section includes many CSS properties that I haven't used before.
Summary
Overall, MDN is a great coding resource. It is important to have a trusted resource when looking for programming help because putting in the wrong code could break your website. No one wants that to happen especially if the website is for a larger business and has lots of visitors.