CSS Normilization is a way to standardize your code and start from a clean slate. By linking this CSS file, all of the CSS properties are reset to the same value every time. This way, you can be sure of exactly how your code is affecting the site, rather than having to work around each individual browser's default values for certain properties.
Semantic markup is the process of using HTML tags to describe the purpose of certain parts of your code. These tags are semantic in the sense that they only exist to add additional details, without directly affecting how the information is displayed (unless specified in the stylesheet). Examples of semantic tags are header, main, and aricle with tell browsers and search engines which parts of your page are the header, main content, and articles, and do not have default CSS properties like b or em.
This image is 600px wide, 337px tall, and takes up 165k of space on the disk.