While many did not, those who did often found that it confirmed what they had learned through trial and error. Learn what practices work in which browsers in order to create pages readable by all browsers. Validation of your work makes this more likely. Understand that copying HTML source from other pages to adapt and practice with is an accepted practice for learning new skills. While all authors should respect copyright, some do not.
Permission is sometimes available for the asking.
Practice, review other pages, practice, review other pages, practice'. There is no single way. Some have attended workshops but they are a minority in the group sampled. The suggestion is that HTML authoring is a set of skills to be learned but not necessarily taught. Common experiences revealed in the survey that have helped develop skills include viewing other authors work subjectively, examining the source code to see how it is done and then finding someone to provide support for any questions.