Correct HTML coding of web pages is vitally important for a variety of reasons, most notably for users and search engines.
HTML Standards

HTML Standards are defined by the World Wide Web Consortium (W3C) and are a set of recommendations on standards for coding with a goal of “web interoperability”, so that web content is compatible across all media and devices.
By coding to these standards we are ensuring the websites that we create display correctly for our users on current and future platforms.
Importance of Semantic Coding
Semantics is the study of meaning. All content on a web-page is surrounded by HTML tags – some of these tags have meaning (h1, h2, h3, p, li, etc) and some do not (div, span). However all can be styled to look the same using CSS (style sheets).
Semantic use of these tags in HTML coding informs the browser (and helps visually impaired users) and search engines as to the meaning of the content within those tags.
An example is:
<h1>My Business Name</h1>
H1 is the tag to indicate a Level 1 Heading – ie, the heading for the entire page. Search engines will pay particular attention to the content within an h1 tag.