Julian Reschke asked on HTTP Working Group mailing list if the character backslash "\" was used frequently inside Etags values.
How the HTTP ETags are used in the wild? So I downloaded the Top Alexa sites list and created a very quick dirty shell script to check on the first 500. This is not a reliable survey, but just to give a quick overview. The results are still surprising.
39 sites have an "etag" including (note the spelling mistake)
On these 39 sites,
3 not using double quotes at all
- 1 being
\ none using the char backslash "\" \