How to get unbroken table border with apache FOP

I'm trying to create a simple Table with solid borders using apache FOP, but the corners are disjoint. How can I get the table borders completely solid?
This is what the top left-hand Table Cell looks like, magnified 64 times:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"  xmlns:fo="http://www.w3.org/1999/XSL/Format"  version="3.0">
    <xsl:template match="XpathNode">
        <fo:root>
            <fo:layout-master-set>
                <fo:simple-page-master  master-name="singlePage"  page-width="297mm"  page-height="210mm">
                    <fo:region-body  margin-left="12.69mm"  margin-top="17.70mm" />
                </fo:simple-page-master>
            </fo:layout-master-set>
            <fo:page-sequence  master-reference="singlePage">
                <fo:flow  flow-name="xsl-region-body">
                    <fo:table>
                        <fo:table-column column-width="2mm"/>
                        <fo:table-column column-width="7mm"/>
                        <fo:table-body>
                            <fo:table-row height="2mm">
                                <fo:table-cell border="0.51mm solid"><fo:block></fo:block></fo:table-cell>
                                <fo:table-cell border="0.51mm solid"><fo:block></fo:block></fo:table-cell>
                            </fo:table-row>
                            <fo:table-row height="4mm">
                                <fo:table-cell border="0.51mm solid"><fo:block></fo:block></fo:table-cell>
                                <fo:table-cell border="0.51mm solid"><fo:block></fo:block></fo:table-cell>
                            </fo:table-row>
                        </fo:table-body>
                    </fo:table>
                </fo:flow>
            </fo:page-sequence>
        </fo:root>
    </xsl:template>
</xsl:stylesheet>

Since you seem to think HTML is acceptable, let see what happens with HTML. Here is a sample directly from the spec, formatted save the HTML to PDF.

As you can see, the top border is square but the side borders are polygons. The top border is not aligned perfectly and in fact overlaps to the left and right of the side borders.

And that is apparent at just about all zoom levels, this is only 1200% which is much less than the 6400%.

Word does things a bit different, but that also has issues. See this at only 200% zoom, you can clearly see the overlap of the red and the black. You possibly don't think this exists because you have all black border.

As you stated, possibly SVG which would likely map to a rectangle in PDF could be a solution. You can use background-image and use base64 encoded SVG inside it or use fo:instream-foreign-object to inline it (but that would depend on whether you want content inside it.

An alternative option would be a black background behind a white <smaller) background, not using any borders. Setting proper padding could yield two rectangles, one behind the other.