The mistake with the namespace syntax is that they wanted to allow plain textual copy-and-paste to work, thus the different ways and places in the document where namespaces can be declared.

The big thing we missed was better tools from the get-go, specially xml-aware text editors. That’s why the editors of the XML specification wanted to make things a bit easier for people doing things like writing XML with a plain text editor, or line-oriented processing using Perl. I think the amount of grief we are stuck with because of those time-limited needs is not worth it, thus the word “mistake” in the beginning of this post.

In hindsight, if anyone asked me how to add namespace support for XML, I would have required them all to be declared at the root of the document, in the XML declaration:

    <?xml version="1.0" standalone="yes"
          xmlns="https://docbook.org/ns/docbook"
          xmlns:m="http://fake-w3.org/2001/Math/not-the-real-MathML"?>
    <book>
        ...
        <equation>
            <title>MathML example</title>
            <m:math mode="display">
                <m:row>
                    <m:underover>
                        <m:o>∑</m:o>
                        <m:row>
                            <m:i>x</m:i>
                            <m:o>=</m:o>
                            <m:i>a</m:i>
                        </m:row>
                        <m:row>
                            <m:i>b</m:i>
                        </m:row>
                    </m:underover>
                    <m:sqrt>
                        <m:i>x</m:i>
                    </m:sqrt>
                </m:row>
            </m:math>
        </equation>
    </book>

Additionally, I would require them to be unique: only one prefix per namespace. That would eliminate a lot of issues when writing XML processors, including editors.

I guess someone else mentioned this or a very similar idea at the time, and it was discarded to support plain text copy-and-paste by having the xmlns attribute in the element being copy-and-pasted. I don’t claim I would have done a better job, but I wish for a new version of XML that learns from these two and a half decades of use.