With HTML4, developers used their own favorite attribute names to style page elements:
header, top, bottom, footer, menu, navigation, main, container, content, article, sidebar, topnav, ...
This made it impossible for search engines to identify the correct web page content.
With HTML5 elements like: <header> <footer> <nav> <section> <article>, this will become easier.
According to the W3C, a Semantic Web:
"Allows data to be shared and reused across applications, enterprises, and communities."