Reading HTML online is generally a better experience than reading a PDF online. With good HTML, the user can choose a font size so the text is easy to read, and the paragraphs are laid out to match the user's preference.
Captions are essential for those who cannot hear or have difficulty hearing the audio. They also help non-native English speakers who may find reading easier to comprehend. Video search results with captions also rank higher than those without captions (auto-captions do not get indexed by Google and therefore do not help Search Engine Optimization).
Foreground (text) against background colours must provide enough contrast to allow colour blind users or those using a monochrome screen to view information. A good contrast checking tool is WCAG Contrast checker (Firefox extension).