Browsers are different, one shows buttons wider, another one makes the site "slide" down, up, left, right. normalize.css is needed so that everything looks the same. As for me, it saves time and nerves when setting up the design.
Semantic markup is when HTML doesn't look like a mess of divs, but like normal text with titles. We have tags that immediately tell us what part of the site is in front of us: header, main, footer, etc. This makes the code easier to read, and makes it easier for other people to understand. And the site becomes more accessible to everyone.
