3.7.1.1 Attributes
1. Each consecutive sequence of literal characters in the attribute content is treated as a string containing those characters, with the following exceptions:
  ...
  c. Each occurrence of two consecutive " characters
     is replaced by a single " character.
  d. Each occurrence of two consecutive ' characters
     is replaced by a single ' character.

Note that these two points apparently both happen regardless of the delimiter. E.g., given:
    <e a="""''">
the value of the constructed attribute node would consist of a single " character followed by a single ' character. I'm pretty sure this was never our intent.

[The wording of points 1c and 1d arrived in erratum XQ.E13:
    http://www.w3.org/XML/2007/qt-errata/xquery-errata.html#E13
which was the fix for Bug 5083. However, the wording proposed in 
Bug 5083 comment 4 did not have the above problem. E.g., it says that
an EscapeQuot is replaced by a quotation mark, and the EBNF says that
an EscapeQuot can only occur in a quot-delimited DirAttributeValue.]