h1 { }
Selects an HTML tag/element.
* { }
Selects everything in the document (or inside the parent element if it is being chained together with another element and a descendant combinator, for example).
.box { }
Matches elements based on the contents of their class attribute.
#unique { }
Matches an element based on the value of its id attribute. In order for the element to be selected, its ID attribute must match exactly the value given in the selector.
a[title] { }
p[title="myTitle"] { }
Matches elements based on the presence or value of a given attribute.
p:first-child { }
a:hover { }
Specifies a special state of the selected element(s).
p::first-line { }
Lets you style a specific part of the selected element(s).
h1 p { }
Matched by the second selector are selected if they have an ancestor (parent, parent‘s parent, etc) element matching the first selector.
h1 > p { }
Matches only those elements matched by the second selector that are the direct children of elements matched by the first.
img + p { }
Matches the second element only if it immediately follows the first element, and both are children of the same parent element.
img ~ p { }
Matches the second element only if it follows the first element (though not necessarily immediately), and both are children of the same parent element.
原文：https://www.cnblogs.com/byeyear/p/11945507.html