What happens if we write invalid HTML markup?

The browser will attempt to correct errors based on HTML5 parsing algorithms. It will create a proper DOM tree, ignoring or correcting improperly nested tags, missing closing tags, and other syntax errors. However, the result can be unpredictable and may vary across different browsers.

Consequences:

• Unexpected appearance: Elements may display incorrectly, affecting layout and styles.
• JavaScript issues: Scripts may not work correctly or fail to run if they depend on a correct DOM structure.
• Accessibility: Users with disabilities may face difficulties using the site.
• SEO: Search engines may index the page less effectively.
• Debugging: The process of finding and fixing errors becomes more complex.

Example of invalid markup:

<div>
  <p>This is a paragraph</div> <!-- Missing closing </p> tag -->

The browser will most likely close the <p> tag automatically, but this is not guaranteed in all cases.

It is recommended to use HTML validators (e.g., from W3C) to check and fix errors.