Automated programs called Robots or Spiders will visit a website & scan it for information to index. When the information is indexed, it can then show up in a Search Engine Results Page (SERP) depending on many factors as to how well it competes. If the Robot gets to a website with poorly formatted code, they will find errors, may skip content or simply time out before it can get to the content.
HTML5 is the current st&ard after 20 years. This new st&ard has two factors for any website's benefit: A future ready code base for newer browsers, &, HTML tags that help Search Engines underst& the content better.
There are many types of Web Browsers & they are constantly evolving & shifting user base %. Firefox, Internet Explorer, Chrome & Safari are among the most popular & they all work differently. Each vendor & each version will look at the same piece of HTML & simply choose to do something different with it. And then there are device types, computers, mobile devices & smart phones. Making them all play well together can be tricky.