"HTML Full-text" means that the complete text of an article is available in HyperText Markup Language, the sort of screen text that you would see on any normal website. You can certainly use HTML Full-text articles in a paper, as long as the source is reliable and is cited properly. When given a choice between HTML and PDF (Portable Document Format), it is best to choose PDF because it is the actual scanned image of the book, journal, or newspaper, and preserves the illustrations, photos, and pagination of the original paper document.