1. HTML is the primary format for articles. Authors can do whatever tricks they like in CSS, but are encouraged to make their layouts reasonably responsive. JS should be limited to things that actually benefit the article (such as LaTeX rendering or simple live examples).
2. The article must fit on one page when printed to PDF using a mainstream Web browser.
3. The author should provide a PDF file. It could just be the output of printing the HTML to PDF from a browser, or it could be something fancier, as long as it fits on a page and has the same text as the HTML version.
Looking at the most recent issue, most articles could be faithfully reproduced with a typical modern Markdown implementation (one that supports tables and code highlighting, and maybe LaTeX math) and some simple CSS.