Pinch-zoom doesn't work if the web site developer has defined the following meta tag on the web page:
user-scalable = no
The above tag may be in use e.g. on mobile optimised web pages and it disables the zooming by the user. The tag is supported by the newer browser versions, e.g. Symbian Anna (and later) and MeeGo browsers. According to the specifications, the user is not allowed to scale the page in the above situation.