The source of this is:

<FORM name="illustration">
    This page shows an example of how to enable or disable a control<p>
    <INPUT name="display" type="text" value="initial" onclick="value='clicked'">
    Click here to disable the text field: 
    <BUTTON name="push" onclick="display.disabled=true">DISABLE</BUTTON>
    (watch the text field fade out)
    Click here to enable the text field:  
    <BUTTON name="push" onclick="display.disabled=false">ENABLE </BUTTON>
</FORM>