1. A method comprising: retrieving, by a computing system, an Extensible Markup Language (XML) tag from a web resource in response to a request for the web resource; matching, by the computing system, the XML tag to a tag in a data type definition (DTD) file to obtain information in the DTD file identifying a first class file for use in rendering a user interface element for the web resource; mapping, by the computing system, the first class file within a servlet based on the obtained information; performing a first rendering, by the computing system, of the user interface element for the web resource by executing program code in the servlet based on the first class file; changing, by the computing system based on user input, the class file identified by the DTD file for the tag from the first class file to a second class file, wherein the DTD file is accessible using XML tags of multiple different web resources; receiving a second request for the web resource; matching, by the computing system, the XML tag to the tag in the DTD file to obtain information identifying the second class file; and performing a second rendering, by the computing system, of the user interface element for the web resource for the second request by executing program code based on the second class file, wherein the second rendering uses a different format for the user interface element than the first rendering.