Working on source development locally.
The above image is HTML source for the 2nd image below after it has been converted to a document readable on a browser.
Types of Source
Including DOM's with backward compatability
DOM also known as the Document Object Model is defined by W3C as of 13 November 2000: "The Document Object Model (DOM) is an application programming interface (API) for valid HTML and well-formed XML documents. It defines the logical structure of documents and the way a document is accessed and manipulated." The link describes the model or tree structure much better than anything I could attempt.
As I attempt to learn source, so far, there are three rules I am trying to keep in mind. First to write in a structure that my computer understands and other people can understand how to read what I have written. My computer needs to understand what I have written as well as other "people" needing to be able to read what is written. This is why we all have taken courses in English... to enable cross communication amongst one another. Secondly, I will need to keep in mind the prior versions of Windows that are in use and to enable the prior versions to understand this version of source. HTML is living and as such it grows and changes. The older models of computers are not living;static machinery does not grow nor learn. Therefore, source must be legible to the older versions while still being readable to the current versions of computers. Finally, and probably the most important factor is accessibility for everyone which I am just beginning to learn. Blind folks have machine readers that need to understand my source as they read my posts. And I am sure not all of the blind can type. Speakers of languages different from my American-English, have translation software to change my writing to their native tongues.