Read the javascript in the headers of Yahoo's login page:
<-- Begin javascript comments from Yahoo -->
/*
* A JavaScript implementation of the RSA Data Security, Inc. MD5 Message
* Digest Algorithm, as defined in RFC 1321.
* Copyright (C) Paul Johnston 1999 - 2000.
* Updated by Greg Holt 2000 - 2001.
* See http://pajhome.org.uk/site/legal.html for details.
*/
<-- End Javascript comments from Yahoo -->
THEY don't even cache, or pass, your password. Like all secure programs,
they store, and transmit, an MD5 Sum.