Div CSS is compatible with Ie and Firefox_html/css_WEB-ITnose

Recently I am writing a website homepage with Div Css layout. The pages I wrote before all have the same width: 900px; there is no layout or float setting. There is nothing we can do now! To cut the page into chunks. Fortunately, I have some basic knowledge of CSS, but it is quite frustrating to use. Under IE and Firefox, the same attribute values ​​often have different display effects. There is no way to make it compatible with browsers, so I have to find bugs one by one. However, it is impossible to be fully compatible with browsers, so we can only show a rough effect, and don't make the deviation too big or obvious.
First of all, what needs to be mentioned is the floating attribute of float, which is the key to div css layout. float:left;float:right; are commonly used floating properties. In order for the divs to be arranged in a row, they must not be used. hehe! Isn't it just a float? What's there to mention? If you want it to float, I'll just float it. Why! That's what I said, but something went wrong when I actually used it. Under IE, as long as the previous div has float:left; the width of the following div does not exceed (the width of the total body) - (the width of the previous div), the following div will automatically float and be arranged on the same line. Under IE and Firefox, this effect is the same. Okay, then go down and lay out the next row in the same way, just divide it into two columns.

< style type ="text/css" >
#div1{}{width:200px;height:80px;float:left;border:1px solid blue;}
#div2 {}{width:600px;height:80px;border:1px solid blue;}

< div >
< div id ="div1" >< / div >
< div id ="div2" >

< div style ="clear:both;margin-top:20px ;" id ="div3" >

Such a layout has different display effects under IE and Firefox. The margin-top under Firefox :20px; has no effect. . . .
The margin-top of div3 works under IE, but not under Firefox. Why? I thought about it over and over again, but finally couldn't figure it out. There was nothing I could do. Then I changed the css. By chance, I added the float:left attribute to div2, and then margin-top:20px had an effect. Depressed, haha. But I still don’t understand why this is happening. Once the problem is solved, it will be ok!
The next question is about ul. Note that the effect of ul{margin-left:10px;} is different under IE and Firefox, so that the effect of the setting will be the same
ul{padding:0;margin-left:10px;} is the default under Firefox The padding!=0, so you need to set it like this...
Haha, stupid personal opinion!