If every project started to make their README filled with HTML (gah, why does Markdown allow arbitrary HTML...), I'd end up crying on the CLI as I frantically search for how to build the project in a sea of <p>, <a> and <br/>s...
Quite frankly I was happy with the arbitrary html part more often than it annoyed me. E.g. when I had to add a table that was just slightly out of markdowns comfort zone (merged cells).
Those who read my readmes were always happy with the content and never complaines about the html.
If every project started to make their README filled with HTML (gah, why does Markdown allow arbitrary HTML...), I'd end up crying on the CLI as I frantically search for how to build the project in a sea of <p>, <a> and <br/>s...