People are excited by reading attractive parts of a book or an article. Some pieces might be very important. Some articles are too long or not easy to understand all pieces firstly.
Most of the time it is not necessary to read entire a blog or a document. We markup some elements which they are more important to us like dates, quotes, key words, thesis statement, conclusion or some phrases. Also, it helps the other people to understand better and faster main idea and concepts.
Markups behave like markers or highlighters. They draw attentions to themselves at the beginning of reading. By dragging or clicking mouse on some digital markups, they toggle and give some hints (XML), or navigate to another part of the same page (as bookmark) or forward to the other web pages (hyperlinks). However, if they are not preserved well or at all in markup, it challenges people by wrong action, ineffective page navigating or incorrect information.
A markup language can be your digital marker. But what's difference between a markup language and a marker? A marker is visible for everybody. You just need highlight some phrases in the book or article. But, a digital marker or better to say "a markup language" not visible to anybody. Do you know why? Because people use different tools and browsers. Also, some browsers can’t open and display mark-up languages properly. Furthermore, brewers are updating occasionally. Do not be scared... You just need to follow World Wide Web Consortium (W3C) instructions and keep update. All standard browsers and XML editors follow same instructions. For example, how to learn/use Extensible Markup Language (XML)? I would suggest following the instructions here: https://www.w3.org/TR/REC-xml/. It is your responsibility, to update your mark-up languages. Some browsers don't open old mark-up languages or even block them (SSL).
There are also some tools can validate your XML and DTD together. XSD (XML Schema Definition) is a World Wide Web Consortium (W3C) and is one of the most popular xml validators. So, if you are worry about your XML and DTD structure I recommend you to learn and use XSD. See https://www.w3.org/XML/Schema
If you want to create a hyperlink you should be more careful. Some browsers block popups or URLs navigate to different Web Protocols (i.e https to http)