One of the problem with the HTML ComboBox is that it does not provide any facility related to autolookup. An example of autolookup and editiable comboBox can found at http://www.personal.psu.edu/users/r/j/rja171/AutoLookup/test.htm
I have extended the original code so as to include an option of button and display all the choices in a comboBox rather than in a layer.
Following are the features of comboBox
1. Allows to type text as in text box
2. At text is being typed shows the drop down list of all available options
3. Allows to browse all the results using a button
4. Selecting a value in the comboBox updates the text.