If the version of HTML that you use is important for your web server, remember that ODS HTML creates HTML 4.01 compliant tags, ODS HTML3 creates HTML 3.2 compliant tags and ODS MSOFFICE2K creates Microsoft HTML, designed for use with Office 97 and higher.
Pull-down menus are implemented in HTML (usually) as HTML forms. The pull-down list is a select box and then when the user makes their selection, the item they select gets posted with the form action.
From other posts you've made, I remember that you are working in an EG and possibly also BI server environment. If you are working with the BI Platform, the SAS/IntrNet examples mostly correspond to what you can implement with the SAS Information Delivery Portal. The Stored Process Web Application (SPWA) is another way that you could create a custom front end and then build whatever kind of prompting interface you want, you would invoke your stored process via URL making a call to the SPWA to execute the stored process.
If you want to implement pull-down menus with EG, you should investigate the Enterprise Guide Parameter Manager -- which lets you define parameters (such as procedure options or task/wizard options) and then the Parameter interface automatically will prompt the user with a pull-down menu when they run a project in which the Parameter Manager was used to define parameters.
Other methods of creating pull-down menus, specifically in EG, involve the use interfaces programmed using methods other than HTML forms generally using custom tasks written in VB.NET or C#.