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"<a href..." links that have single quotes instead of double quotes are NOT crawled by Google.
When run thru the HTML checker - it's OK and of course it works for visitors. However, the spider sim notices the problem. (If I looked closer I would have noticed some of my links were missing).
I suppose if Google keeps moving in that direction, it could result in a new kind of hidden link, e.g., a javascript link that doesn't actually work for humans.
3.2.2 Attributes [w3.org]
By default, SGML requires that all attribute values be delimited using either double quotation marks (ASCII decimal 34) or single quotation marks (ASCII decimal 39).
I've never used single quoted attributes except in the case of javascript. I've not seen much discussion on this either. I quote all attributes as this is required to pass validation.
In general, using double quotes is preferable, since for the human eye single quotes are sometimes difficult to distinguish from other characters like accents. Moreover, some browsers do not handle single quotes correctly.
In html 3.2, double quotes were mandatory. Single quotes were not valid. I believe single quotes are a part of the 4.0 standard. Most of what I've read recommends that you use single quotes only where applicable. Typically quoted objects within quotes.
<added>Sorry dmorison, I didn't see your reference to the W3C up above. ;)