Some of the attributes I liked in HTML5

Reduces N number of javascripts and CSS with the below cool attributes in HTML5

placeholder :- lets you add a hint inside the textfield, but where the hint automatically disappears when you click inside it.
pattern :- lets you specify a regular expression (in JavaScript RegEx format) for legal values
required :- it specifies that an input field must be filled out before submitting the form.
autofocus :- element should automatically get focus when the page loads.
list :- refers to a element that contains pre-defined options for an element.
hidden :- it specifies that an element is not yet, or is no longer, relevant.
contenteditable :- specifies whether the content of an element is editable or not.
async :- can be used along with script. allows you to run the script asynchronously as soon as it is available

More details can be found in the below links