# HTML && CSS

1. The Div is centered left and right

<div>1<div>div{
width；300px;height:300px;margin:0 auto;
}

2. The Div is centered up, down, left, and right

<div class=&#39;father&#39;>
   <div class=&#39;son&#39;></div></div>.son{position:absolute;top:50%;left:50%;width:300px;heigeht:300px;
margin-top:-150px; /*等于高度的一半*/margin-left:-150px; /*等于宽度的一半*/}

3. The background image is centered up, down, left, and right

background-position:center;

4. The border becomes a small triangle
We get a small triangle with a width of 10px, a black downward direction, and the background The color is red

div{
width:0;
height:0;
border:10px solid red;
border-top-color:black;
}

5. About line wrapping

white-space:nowrap;/*不换行*/
white-space:normal;/*换行*/

6. Regarding the transmission problem of margin-top
It can be solved by adding a border to its parent element

7. Clear the default style

body,h1,h2,h3,h4,h5,h6,dl,dd,p{marhon:0;}
ul,ol{margin:0; padding:0;}
th.td{padding:0;}

8. Table

<table border=&#39;1&#39;>
<thead><tr><th>sex</th><th>age</th></tr></thead>
<tbody>
<tr><td>man</td><td>18</td></tr>
<tr><td>female</td><td>28</td></tr>
</tbody>

table{border-collapse:collapse;}
跨行合并：rowspan;
跨列合并：colspan;

9. Form

/*for和id相同*/<label for=&#39;username&#39;>用户名</lable>
<input type=&#39;text&#39; id=&#39;username&#39; value=&#39;Input your name&#39;/>

10. Clear float

.clearfix:after{
display:block;
content:'';
clear:both;
}
.clearfix{
zoom:1;  /* for IE6*/
}