HTML is the front page our any website. All the fonts and content you see in the pages of a website is written in HTML format. All the browsers will have the default HTML scripts installed in it. Every browser supports HTML formats. You can simple code HTML documents from your notepad. You just need to add .html at the end of your file while saving it.
CSS is used to design the styles of a website. Styles such as theme, color, Widgets, Menus, footers and all the other designs are done with CSS. We can consider this as frame work of a website. This is where you can design how your website should look.