That is, spaces before the slash in XML empty-element tags. E.g. <foo attr="val" />, as opposed to <foo attr="val"/>.

This habit originates in guideline in the XHTML1 spec. It's a hack to get old browsers that only know about HTML to accept XHTML1. But it only addresses the issue of the difference between XML empty-element tags and SGML empty element types, and even then it is dubious (see about halfway through this document). There are many other more subtle differences between XHTML1 and HTML.

But even if one accepts this custom in the context of XHTML1, it has no sensible place in XML documents that are not XHTML. But I have seen it infecting XML documents in general.