HTML5 and CSS3 are very old now but they still have to be use very carefully because millions of users still using old browsers that does not support them.
Sometimes this seems awful.

Everyone’s using it, nobody knows what it is. I realize that sounds more like a line out of an existential movie — maybe Waiting for Godot or a screenplay by Sartre — than a statement about HTML5. But it’s really the truth: most of the people using HTML5 are treating it as HTML4+, or even worse, HTML4 (and some stuff they don’t use). The result? A real delay in the paradigm shift that HTML5 is almost certain to bring. It’s certainly not time to look away, because by the time you look back, you may have missed something really important: a subtle but important transition centered around HTML5.

Introduction of an article, another section or the entire document (header

page). Typically the header of a web site that appears on top of each

page, or a header of a long

or of a long

Contains the footer of a site, a long

, or a long

Section that contains the main navigation links (within the document or to

other pages).

Independent content, which can be individually extracted from the

document and syndicated (RSS or equivalent) without penalizing its

understanding. Typically a blog post.

Generic section used to group different articles into different purposes or

subjects, or to define the different sections of a single article. Generally

used with a header.

Used for marking up times and dates.

Section whose content is not necessarily directly related to the main

content that surrounds it, but can provide additional information.

Used to wrap more than one heading if you only want it to count as a

single heading in the page's heading structure.

and

Used to encapsulate a figure as a single item, and contain a caption for

the figure, respectively.