When we have a tbody tag inside a unclosed th then the output is different from the way chrome renders it.
While closes the entire thead, HAP adds the tbody tag inside the th.

HTML: <table><thead><tr><th></th><th></th><th><tbody></table>
HAP Result: <table><thead><tr><th></th><th></th><th><tbody></tbody></th></tr></thead></table>
Chrome Result: <table><thead><tr><th></th><th></th><th></th></tr></thead><tbody></tbody></table>

This is the test I used:

var html = @"<table><thead><tr><th></th><th></th><th><tbody></table>";
var doc = new HtmlDocument();
doc.LoadHtml(html);
var newHtml = doc.DocumentNode.OuterHtml;  //<table><thead><tr><th></th><th></th><th><tbody></tbody></th></tr></thead></table>
var res = @"<table><thead><tr><th></th><th></th><th></th></tr></thead><tbody></tbody></table>"; //chrome or edge rendering results
var b = newHtml == res; //false