Cascading Style Sheets (CSS) is a design language for styling HTML documents. In modern websites, HTML controls the structure while CSS controls its presentation. Let's suppose that you have some HTML formatting in numerous pages across your website. Would you want to manually modify 10,000 HTML elements to implement the new formatting? Ideally, there should be one place where you can make a change that modifies that formatting everywhere you need in the website. Technical term for this concept is "modular design". This technology already exists and it is called CSS.
HTML5 offers many new input types which were not available in previous versions. Smart phones and tablets have limited screen real estate and the interfaces need to be sensitive to touch which requires more space than a mouse pointer. These interfaces require users to switch between keyboards to input characters. HTML5 forms allow web designers to make their sites more user-friendly by offering preselected keyboard such as a date picker for date field, @ and . symbols in the keyboard for emails, and numeric keyboard for numbers.
The web is the most popular feature of the Internet and HTML is the backbone of the web. Everything that we see on the web is used with HTML. All fancy web technologies either generate or manipulate HTML in different ways.