When I learned HTML tables back in the 90s, at some point I discovered the <thead> element for grouping the <th> column headers. What I missed was there should be a <tr> element between the two. In other words, a well-formed HTML table with a header looks like this:

<table>
    <thead>
        <tr>
            <th>Name</th>
            <th>Value</th>
            <th>Date</th>
        </tr>
    </thead>
    <tbody>
        <tr>
            <td>USERNAME</td>
            <td>John.Smith</td>
            <td>2017-02-18T23:47</td>
        </tr>
    </tbody>
</table>

and not:

<table>
    <thead>
        <th>Name</th>
        <th>Value</th>
        …

The latter form—<thead> directly enclosing <th>s—had always worked for me. Until yesterday when I ran afoul of an HTML5 validator on a remote API, which simply would not let me proceed until I wrapped my <th> cells with a <tr>.

Who knew?