1) what is the doctype.......why it is required. if i do not provide doctype then what will be the disaster ?
<!DOCTYPE html>
<!--[if lt IE 7]> <html class="no-js lt-ie9 lt-ie8 lt-ie7"> <![endif]-->
<!--[if IE 7]> <html class="no-js lt-ie9 lt-ie8"> <![endif]-->
<!--[if IE 8]> <html class="no-js lt-ie9"> <![endif]-->
<!--[if IE 9]> <html class="no-js ie9"><![endif]-->
<!--[if !IE]> --> <html class="no-js"> <!-- <![endif]-->
2) when below statement people use
<!--[if lt IE 7]> <html class="no-js lt-ie9 lt-ie8 lt-ie7"> <![endif]-->
what the above statement is doing ?