p { }             /* element selector */
.card { }         /* class selector */
#header { }       /* id selector */
[type="text"] { } /* attribute selector */
p.intro { }        /* combined selectors */
.card > p { }      /* direct child combinator */
.card p { }         /* descendant combinator */
p:hover { }         /* pseudo-class */
p::before { }        /* pseudo-element */