css属性在没有设置的时候，也是有值的，这就是默认值。列在下面，方便查看。

元素	默认的 CSS 值
a:link	color: (internal value); text-decoration: underline; cursor: auto;
a:visited	color: (internal value); text-decoration: underline; cursor: auto;
a:link:active	color: (internal value);
a:visited:active	color: (internal value);
abbr	None.
address	display: block; font-style: italic;
area	display: none;
article	display: block;
aside	display: block;
audio	None.
b	font-weight: bold;
base	None.
bdi	None.
bdo	unicode-bidi: bidi-override;
blockquote	display: block; margin-top: 1em; margin-bottom: 1em; margin-left: 40px; margin-right: 40px;
body	display: block; margin: 8px;
body:focus	outline: none;
br	None.
button	None
canvas	None.
caption	display: table-caption text-align: center;
cite	font-style: italic;
code	font-family: monospace;
col	display: table-column;
colgroup	display: table-column-group
datalist	display: none;
dd	display: block; margin-left: 40px;
del	text-decoration: line-through;
details	display: block;
dfn	font-style: italic;
dialog	None.
div	display: block;
dl	display: block; margin-top: 1em; margin-bottom: 1em; margin-left: 0; margin-right: 0;
dt	display: block;
em	font-style: italic;
embed:focus	outline: none;
fieldset	display: block; margin-left: 2px; margin-right: 2px; padding-top: 0.35em; padding-bottom: 0.625em; padding-left: 0.75em; padding-right: 0.75em; border: 2px groove (internal value);
figcaption	display: block;
figure	display: block; margin-top: 1em; margin-bottom: 1em; margin-left: 40px; margin-right: 40px;
footer	display: block;
form	display: block; margin-top: 0em;
h1	display: block; font-size: 2em; margin-top: 0.67em; margin-bottom: 0.67em; margin-left: 0; margin-right: 0; font-weight: bold;
h2	display: block; font-size: 1.5em; margin-top: 0.83em; margin-bottom: 0.83em; margin-left: 0; margin-right: 0; font-weight: bold;
h3	display: block; font-size: 1.17em; margin-top: 1em; margin-bottom: 1em; margin-left: 0; margin-right: 0; font-weight: bold;
h4	display: block; margin-top: 1.33em; margin-bottom: 1.33em; margin-left: 0; margin-right: 0; font-weight: bold;
h5	display: block; font-size: .83em; margin-top: 1.67em; margin-bottom: 1.67em; margin-left: 0; margin-right: 0; font-weight: bold;
h6	display: block; font-size: .67em; margin-top: 2.33em; margin-bottom: 2.33em; margin-left: 0; margin-right: 0; font-weight: bold;
head	display: none;
header	display: block;
hr	display: block; margin-top: 0.5em; margin-bottom: 0.5em; margin-left: auto; margin-right: auto; border-style: inset; border-width: 1px;
html	display: block; html:focus outline: none;
i	font-style: italic;
iframe:focus	outline: none;
iframe[seamless]	display: block;
img	display: inline-block;
input	None.
ins	text-decoration: underline;
kbd	font-family: monospace;
label	cursor: default;
legend	display: block; padding-left: 2px; padding-right: 2px; border: none;
li	display: list-item;
link	display: none;
main	None.
map	display: inline;
mark	background-color: yellow; color: black;
menu	display: block; list-style-type: disc; margin-top: 1em; margin-bottom: 1em; margin-left: 0; margin-right: 0; padding-left: 40px;
menuitem	None.
meta	None.
meter	None.
nav	display: block;
noscript	None.
object:focus	outline: none;
ol	display: block; list-style-type: decimal; margin-top: 1em; margin-bottom: 1em; margin-left: 0; margin-right: 0; padding-left: 40px;
optgroup	None.
option	None.
output	display: inline;
p	display: block; margin-top: 1em; margin-bottom: 1em; margin-left: 0; margin-right: 0;
param	display: none;
picture	None.
pre	display: block; font-family: monospace; white-space: pre; margin: 1em 0;
progress	None.
q	display: inline;
q::before	content: open-quote;
q::after	content: close-quote;
rp	None.
rt	line-height: normal;
ruby	None.
s	text-decoration: line-through;
samp	font-family: monospace;
script	display: none;
section	display: block;
select	None.
small	font-size: smaller;
source	None.
span	None.
strike	text-decoration: line-through;
strong	font-weight: bold;
style	display: none;
sub	vertical-align: sub; font-size: smaller;
summary	display: block;
sup	vertical-align: super; font-size: smaller;
table	display: table; border-collapse: separate; border-spacing: 2px; border-color: gray;
tbody	display: table-row-group; vertical-align: middle; border-color: inherit;
td	display: table-cell; vertical-align: inherit;
template	None.
textarea	None.
tfoot	display: table-footer-group; vertical-align: middle; border-color: inherit;
th	display: table-cell; vertical-align: inherit; font-weight: bold; text-align: center;
thead	display: table-header-group; vertical-align: middle; border-color: inherit;
time	None.
title	display: none;
tr	display: table-row; vertical-align: inherit; border-color: inherit;
track	None.
u	text-decoration: underline;
ul	display: block; list-style-type: disc; margin-top: 1em; margin-bottom: 1 em; margin-left: 0; margin-right: 0; padding-left: 40px;
var	font-style: italic;
video	None.
wbr	None.

小结

尽量记住这些默认值。