Both Microsoft IE 5+ and Netscape 6.0 have the "autocompleted" feature, which store previously entered field information such as username or password on the browser. This presents an "opportunistic" security concern where users move from computer to computer. Without being a hacker, a user can stumble on a another user name (and try and guess the password) or worse have the password also appear just by entering the first few letters in the username field of the Domino login form. As opposed to changing every browser setting to disable this feature (admin nightmare to enforce), on the customized Domino login form just add the html field attribute AUTOCOMPLETE="OFF" to both the username field and the password field.
When looking from the web, the HTML would look like:
<input type="text" name="username" AUTOCOMPLETE="OFF">
<input type="password" name="password" AUTOCOMPLETE="OFF">
For more information on Autocomplete check