Have you ever seen usage of <span> like this?

<span content="2010-01-08 21:35:12" property="开发者_如何学Pythondc:date">

What does it mean?

It seems to be XHML with Dublin Core metadata, a set of metadata field standards.

In HTML, Dublin Core info is used in meta and link elements only, and I can not find any instance where the data is validly used in a span element. Also, the content attribute is not valid in HTML. See Expressing Dublin Core in HTML/XHMTL meta and link elements.

The case is different with XHTML: As @tomlog points out in his comment, the notation you quote is used in this example on Wikipedia.

Those aren't standard tags, but they are probably used by some javascript on the page that can search based on those properties, or they are akin to comments that the programmer is inserting in the html output.

I would say it appears to be meta-information for whatever goes within the span, or it's storing values for Javascript to use at a later time, or both.

Seeing the "dc" makes me think that there may be more crucial bits that aren't included in your example.

It's a kind of meta data implementation. "dc" stands for Dublin Core which is a meta data implementation standard.

The appropriate software that can read these meta tags will know to look for a span element and then use the property and content attributes to retrieve the relevant information.

property="dc:date" is a Dublin Core Metadata tag of type date. It makes the data in that span, machine readable using RDFa semantics. Google/ other crawlers can read that info and index it appropriately for searching and relating to other documents. You can test a sites metatdata here.

The inclusion of the DC tag in a span is very common.