To properly edit HTML5, every page of content should be wrapped with the <html> tag and the <body> tag.
This next bit is kind of tricky, but you should also include a <header> tag between your <html> and <body> tag. Within the header you should put a <title> tag. This contains meta-data about your web page, and is an important part of SEO. The title will not display on your page, but it should display on your browser tab. It is common for <title> and <h1> tags to contain the same text.
Below is a picture from a text editor to show you a properly edited basic page.
HTML5 Makes Your Page Respond To Different Screen Sizes
Visit some random web pages and play with size of your browser window. Alternatively, view the pages on a desktop and then a smart-phone. Does the page respond to different screen sizes? If it does then you can assume that it was edited in HTML5.
Now take a moment to view the source code of the world’s first web page. You can view the source code and the HTML that was written. Using Google Chrome to view source code you click on ‘Tools’ and select ‘Developer Tools’. Notice that the HTML is a bit different from HTML5. That’s because HTML is a living language that constantly evolves. HTML5 simply represents a point when the W3C decided to mark it as the latest standard for HTML editing.