In previous versions of Microsoft Outlook (the classic app), you could view the HTML code of an email by opening the email, right-clicking on it, and selecting “View source” from the context menu.

Search Engine Land's Guide to SEO Chapter 3: HTML Code & Search Engine Ranking Success Factors explains HTML elements & using structured data.

HTML is not a programming language because it fails to meet several functional criteria. Nevertheless, HTML coding is valuable beyond that pedantic debate.

How to Obfuscate HTML. To obfuscate an HTML code page means to encrypt or hide strings of characters which are likely email addresses and other source code within the code page.