you mean in preview player or after encoding using ffmpeg video is unplayable?
Can you give some more details about your browser, browser version, os, os version and pc/mobile, file, if its publically available that i can replicate. Any other info to debug would be helpful as well. Last but not the least i hope you have pressed play button and waited for some time like 2-3 seconds.
Converting web pages to Markdown is a common requirement. I have found that turndown does a good job, but it cannot meet the needs of all dynamic web page content. As far as I know, if you need to process dynamic web pages, you need targeted adaptation, such as Google extensions such as Web2Markdown.