[html] Why there is this "clear" class before footer?

Most likely, as mentioned by others, it is a class carrying the css values:

.clear{clear: both;} 

in order to prevent any more page elements from extending into the footer element. It is a quick and easy way of making sure that pages with columns of varying heights don't cause the footer to render oddly, by possibly setting its top position at the end of a shorter column.

In many cases it is not necessary, but if you are using best-practice standards it is a good idea to use, if you are floating page elements left and right. It functions with page elements similar to the way a horizontal rule works with text, to ensure proper and complete sepperation.