This is the link to get to the MDN page that talks all about HTML.
This is the link to get to the MDN page that talks all about CSS.
HTML and CSS are two forms of coding, HTML or Hypertext Markup File and CSS or Cascading Style Sheet. HTML being the code or the basic building blocks of a website. CSS is the styling or the visual aspects of websites. You want to use Html for building the website itself and providing the correct pathways for links you may have on the page. Html is also how you get any information onto a page using specific words or letters to get that information programed into your document. CSS you want to use to change the color or to make everything look nice and visually appealing.
CSS is not so much about putting information onto a page but organizing and changing the way it appears on the page. In CSS you can change the font, color, size and spacing of your content in order to make your page go from dull and boring to something people will actually enjoy and think is nice to look at. These pages are both great for when you need a little extra help when working with HTML or CSS, they also have a lot more info about a range of other programing topics as well. These sites have helped a lot of people and they will continue to help people with programing for years to come.