Yes, it's obviously "against common usage" given HTML support exists specifically for less common features that Markdown does not support. Like tables, which are supported by some implementations but not all, and iirc not even all Markdown variants that support tables use the same syntax for them. The only way to be 100% sure is to use HTML. Of course you wouldn't do that if you just have the file on Github, but in general HTML is supported in Markdown for a reason.