CSS

Cascading Stylesheets (CSS) are used to style websites - i.e. to make them look pretty. Whereas a page without CSS is just displayed as plain content:

One with CSS looks a lot more presentable:

CSS declarations use the tag names (e.g. p), classes and IDs of the elements on the page in order to style them. IDs are written with a preceding hashtag, classes are written with a preceding full stop and tag names are written without any preceding characters. The styles themselves (like height) should all end in semicolons. Here are some example declarations:
p {
    font-size: 3rem;
}
.class_name {
    height: 200px;
}
#id_name {
    font-family: 'Roboto'
}

A comment in CSS is started with /* and ended with */.