Markdown stopped working after I started taking some HTML code snippets

Mark the HTML sections as inline code (single backticks) and you should be fine. e.g.

`<!DOCTYPE html>` defines an HTML5 document. Tells browser to use HTML5
`<html>` is the root element
`<head>` contains info about the page
`<body>` defines the body, only content inside of the body is shown in the browser
`<h1>` defines a large heading
`<p>` defines a paragraph
`<a>` defines a link 
...

Have a look at the gif here: