The mask here is a bit problematic. The height is not all masked, and the width is not all masked when the browser screen is reduced.
<Style> body {margin: 0;}/* body <HTML <document */body, HTML {Height: 100%;}/* compatible with earlier ie versions, to set the body and HTML to 100% */. floats {position: absolute; top: 0; left: 0; width: 100%; Height: 100%; Background: #000; opacity: 0.5; filter: alpha (opacity = 50 );}. alert {width: 400px; Height: 200px; Background: # FFF; Border: 2px solid yellow; position: absolute; top: 50%; left: 50%; margin-top:-102px; margin-left:-202px;}/* If margin-top and margin-left are not added, they are not centered. To add these two attributes, the top value is half of its height, and left is half of its width, because the border is added, two pixels are added. * // * standard opacity: 0 ~ 1; ie private filter: alpha (opacity = 0 ~ 100 ); */</style>