Probably related to bug 15616.

In Section 10.4, "Context" of content element reads:
    "Where flow content is expected."

However, <content> element should be able to appear in a place where non-flow
content is expected. Actually, an example in Section 11 illustrates such cases:

    <div class="breaking">
        <ul>
            <!-- The content model of <ul> is "zero or more li elements",
                 which is NOT flow content. Therefore, the <content> below
                 is non-conformant. -->
            <content select=".breaking"></content>
        </ul>
    </div>

<table> and <tr> have special content model like <ul>, so if <content> appears
directly under these elements, they become non-conformant (which should be
unexpected).

The definition of <shadow> element (in Section 10.5) has the exact same issue.