Browsers are usually not following strict HTML5 rules in order to be robust to syntax errors, and to avoid breaking old style HTML programming. They typically allow incorrect html code to be rendered in a "correct" way, helping non expert developers to obtain quickly visual results.
While, often, rendering correctly in browser, writing non standard code is considered as a bad practice. It may render differently on various browser, and may be rendered incorrectly in the future version of the browser.
A good practice consists in checking your HTML code in validator.

The W3C Validator is a common tool to check that your code follows standard rules (browser extensions are also available).

Check regularly if your HTML code is valid, many typos and errors which are silently rendered by your browser can be detected and cleaned up.