About This HTML Issue
On <svg> and the elements inside it, the xmlns attribute can only hold the SVG namespace, http://www.w3.org/2000/svg.
When the HTML parser reads inline SVG, it places <svg> and its descendants in the SVG namespace by itself, whatever xmlns says. The main exception is content inside <foreignObject>, which is parsed as HTML. Because the attribute can't change the namespace, the HTML standard makes any other value a parse error, whether it sits on the <svg> element or on a child such as <g> or <path>. A trailing slash or the XHTML namespace triggers it just like an unrelated URI does.
Design tools are a common source. Adobe Illustrator can export a <metadata> block holding an <sfw> element declared in its own Save for Web namespace, http://ns.adobe.com/SaveForWeb/1.0/. In a standalone .svg file, which is parsed as XML, that declaration puts <sfw> in Adobe's namespace. Pasted into HTML, the declaration has no effect and <sfw> becomes an unknown SVG element, so the validator also reports sfw and slices as unknown elements. Browsers don't render <metadata>, so deleting the block leaves the graphic unchanged, and SVG optimizers such as SVGO remove it by default.
For the attribute itself, either set it to exactly http://www.w3.org/2000/svg or remove it, because inline SVG in HTML doesn't need it.
Invalid example
<svg viewBox="0 0 24 24" width="24" height="24">
<metadata>
<sfw xmlns="http://ns.adobe.com/SaveForWeb/1.0/">
<slices></slices>
</sfw>
</metadata>
<circle cx="12" cy="12" r="10" fill="teal"/>
</svg>
Valid example
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" width="24" height="24">
<circle cx="12" cy="12" r="10" fill="teal"/>
</svg>
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