title: are custom html attributes without data-* prefix a valid attribute?
body: <p>Okey, so, recently, i found this: <a href="https://angularjs.org/" rel="noreferrer">https://angularjs.org/</a></p>
  
  <p>I noticed that they use custom attribute prefix "ng-"</p>
  
  <p>From articles, like: 
  <a href="http://html5doctor.com/html5-custom-data-attributes/" rel="noreferrer">http://html5doctor.com/html5-custom-data-attributes/</a></p>
  
  <p>or even stackoverflow:
  <a href="https://stackoverflow.com/a/17091848/2803917">https://stackoverflow.com/a/17091848/2803917</a></p>
  
  <p>And there are many more, the only VALID (im not talking about the fact, that they work anyways) prefix to use is "data-".</p>
  
  <p>So, could someone explain to me, how can it be, that these, million projects and companies, uses an invalid prefix for custom html element attributes and no one seems to care?
  Or am i missing something?</p>
  
  <p>I would really appreciate some thoughts, or even sources of info, not just texts like "everyone does it" and "don't bother and leave it".</p>