Modernizr is a JavaScript library that you include on your page that executes itself and adds a series of classes to your HTML tag. This allows to implement modern CSS functionality without worrying about writing conditionals in JavaScript or anything complicated like that. You simply write one (or at most two) style definitions around the functionality you want, like this:
.functionalityYouWant #myElement{
css3thing: blah;
}
.no-functionalityYouWant #myElement{
oldSchoolWay: blah;
}
So, real world example:
.cssgradients .sideNavTitleBox{
background: -moz-linear-gradient(center bottom, #000 13%, #353535 84%);
background: -webkit-gradient(linear, center bottom, center top, color-stop(0.13, #000), color-stop(0.84, #353535));
}
.no-cssgradients .sideNavTitleBox{
background-color: #000;
background-image: url(/media/images/backgrounds/left_nav_box_header.gif);
background-repeat: repeat-x;
background-position: left top;
}
Again, you don’t have to write any javascript at all, you just include the library on the page and it runs all on it’s own and enables this awesomeness!
EDIT: Oh look, ALA just posted a great article on modernizr.
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