limitations of .aspx web pages rendered in OS X Safari
ASPX is a Server Side Choice

ASPX is a server side technology. The web pages created by ASPX are not inherently limited on Safari browsers.

This is the same for other server side technologies such as Java, php, and perl.

Developer's Responsibility

The developer of the .aspx created page is responsible for producing content that conforms to widely accepted web standards. These standards include HTML5 and those published by the World Wide Web Consortium.

Web developers can refer to resources such as QuirksMode for a guide to what features are supported by which browsers.

Webmasters' Stack Exchange

For help with specific web technologies and choices, please ask on the specialised Webmasters Stack Exchange.