Question

I’ve heard that using single quotes to surround XML attribute values is a “bad style”. Is this correct?

Should I always write:

<element attr="value">

Or is it acceptable to write:

<element attr='value'>

Or does it not matter which style I use?

Answer 1

Both are legal. Choose one and stick with it. It doesn’t matter.

From the spec:

AttValue       ::=      '"' ([^<&"] | Reference)* '"'
                     |  "'" ([^<&'] | Reference)* "'"

Showing that both are valid, as is mixing the two styles within an element, per attribute (though I suggest being consistent within any single document/set of documents).