HTML

<table>
    <tr><td class="comment">Foo</td></tr>
</table>

CSS

* { margin: 0; padding: 0;}
td { border: 1px solid black; }
.comment:after {
    content: "";
    position: relative;
    top: 0.5em;
    border-left: 0.5em solid transparent;
    border-top: 0.5em solid red;
}

DEMO

修正包装的更新答案：

/* add relative positioning to the td */
td { border: 1px solid black; position: relative; }
/* and absolute positioning for the triangle */

.comment:after {
    content: "";
    position: absolute;
    /* left: calc(100% - 0.5em); */
    /* use right: 0; instead */
    right: 0;
    top: 0;
    border-left: 0.5em solid transparent;
    border-top: 0.5em solid red;
}

Fixed Demo