CSS (Cascading Style Sheets) are used to control the visual/presentational aspect of web pages, including things such as colors, fonts, margin, padding, and position.
When using CSS to design web pages, it is usually best to have the CSS in separate .css files, since these can be cached by the browser. Files that are cached only needs to be downloaded once, on subsequent requests the file will be loaded from the browser cache rather than downloading it again.
CSS files is usually loaded in the head part of HTML pages, but it is also possible to load them from the body.
When defining font sizes, it is recommended to use relative units such as em and rem.
For element-widths, use a combination of percentages and pixels with the min/max-width CSS properties, and media queries.