The extension ASPX stands for Active Server Pages Extended (ASP.NET) and represents the successor to the Internet technology ASP. This technology developed by Microsoft Corporation is used for the server-side creation of dynamic web pages and has little in common with the old ASP version ,
Since ASPX files are typically executed on a web server, any browser can be used to display the resulting document. Aspx files are similar to PHP or CGI. The users never get to see the actual ASPX file, but only the created HTML page.
ASPX files are just text files that can be processed on an ASP.NET server. They can be opened and edited in any text editor, however it is recommended to use editors with developer functions such as syntax highlighting for easier work.
The extension ASPX is mostly used instead of htm or html on websites, if the server is running under Microsoft IIS and uses ASP.NET. On Linux there is a possibility with mono to develop and host ASP.NET pages on a Linux system (with restrictions).