The javascript: pseudo protocol is commonly used in webpages, yet it is unsafe for a number of reasons. Javascript isn't always enabled or supported, and when it's not, the pseudo protocol obviously doesn't work. The format can confuse users who wish to open the link in a new window for example, this will generally result in either an error, or "incorrect" results. Also a number of javascript capable browsers, don't also support the pseudo protocol, fortunately there are easy solutions, normally involving the onclick event, onclick whilst device dependant in the specification, is implemented such that it's not specific to the mouse in all browsers.
<a href="javascript:window.open('fred.html','windowname','height=100')">Fred</a>
<a onclick="if (window.open) window.open('','windowname','height=100')" href="fred.html" target="windowname" >Fred</a>
<a href="javascript:someFunction('data')">Fred</a>
<a onclick="someFunction('data');return false" href="nojavascriptfallbackpage.html" >Some Function</a>
Using onMouseOver can give good interactivity to pages which enhances accessibility, however it is meaningless if the user doesn't have a mouse. One approach is to duplicate the onmouseover onfocus, current browser implementations (September 2001) have no problems, but it's conceivable that future browsers may fire both events, so it's best to both bear this in mind, and use identical functions so that the User Agent is aware that they do the same thing which will lessen the chance of this.
<a onmouseover="ChangeImage()" href="moomin.html" ><img src="moomin.png" alt="Moomin"></a>
<a onmouseover="ChangeImage()" onfocus="ChangeImage()" href="moomin.html" ><img src="moomin.png" alt="Moomin"></a>
Remember if you're changing the image in the document to change the ALT attribute aswell if need be.
Some current browser implementations (September 2001) fire the onchange event when the keyboard is used to navigate through a select list, therefore, if you use the common technique of navigating to a page onChange of a select box, keyboard users will not be able to use that navigation. A standard list of HTML links is likely better than a <SELECT> list for navigation, or ensure you use something other than onChange. onClick is a possibility, but is not supported by certain older browsers, a seperate control to actually perform the navigation.