Supported web-mails: Yahoo (both classic and new), Gmail, Hotmail and AOL.
Supported browsers: IE6-8, FF3+, Chrome, Safari

Pure javascript (no jquery, prototype, etc.) that will be injected to an HTML page (our already developed plugin will inject your JS code into the page).
The component needs to identify if this page has a "compose"/"reply"/"forward" text area and if that's the case, it should add at the bottom of the text area a fixed HTML string (taken from a const within the JS)
Note: if the const string was found in the text area then we should not add it again.
Note 2: error handling is required. If for some reason there is an error in your JS code then it shouldn't effect the host page.

Task will be completed if you run "javascript:[your JS code]" from the browser address bar and the signature will be added correctly for the supported web-mails but will do nothing for other pages.

For reference you can check WiseStamp firefox extension (http://www.wisestamp.com/) which is much bigger project (multiple signatues, edit and preview your signatures, embed iconc/menus, etc.)