Some very handy classes and selectors to know.
a:link - Selects all unvisited links
a:visited - Selects all visited links
a:hover - Selects links on mouse hover
a:active - Selects the active link element
p:after - Insert content inside element <p>
p:before - Insert content inside element <p>
p::after - Insert content after </p> element
p::before - Insert content before <p≶ element
input:checked - Selects every checked <input> element
input:disabled - Selects every disabled <input> element
input:enabled - Selects every enabled <input> element
input:focus - Selects the <input> element which has focus
input:in-range - Selects <input> elements with a value within a specified range
input:invalid - Selects all <input> elements with an invalid value
input:optional - Selects <input> elements with no ‘required’ attribute
input:out-of-range - Selects <input> elements with a value outside a specified range
input:read-only - Selects <input> elements with the ‘readonly’ attribute specifi
input:read-write - Selects <input> elements with the ‘readonly’ attribute not specified
input:required - Selects <input> elements with the ‘required’ attribute specified
#id:target - Selects the current active
#id element input:valid - Selects all <input> elements with a valid value
p:empty - Selects every <p> element with no children
p:first-child - Selects every <p> element that is the first child of its parent
p::first-letter - Selects the first letter of every <p> element
p::first-line - Selects the first line of every </p> element
p:first-of-type - Selects every <p> element that is the first </p> element of its parent
p:lang(language) - Selects all <p> elements with a lang attribute equal to ‘language’
p:last-child - Selects every <p> element which is the last child of its parent
p:last-of-type - Selects every <p> element which is the last element of its parent
p:only-of-type - Selects every <p> element that is the only <p> element of its parent
p:only-child - Selects every <p> element that is the only child of its parent
:not(p) - Selects every element that is not a <p>
p:nth-child(even) - Selects every second <p> element
p:nth-child(odd) - Selects odd <p> elements
p:nth-child(2) - Selects every <p> element that is the second child of its parent
p:nth-last-child(2) - Selects every <p> element that is the second child of its parent, counting from the last child
p:nth-last-of-type(2) - Selects every <p> element that is the second element of its parent, counting from the last child
p:nth-of-type(2) - Selects every <p> element that is the second element of its parent
:root - Selects the documents root element
::selection - Selects the portion of an element that is selected by a user
