Many many web-based, expository mathematics articles will contain worksheets or source files for mathematical software such as Mathematica, Maple, Matlab, or Excel, to name just a few. For maximum accessibility, an author should use text-based, open-source formats when this can be done without harming the mathematical exposition. When open source formats are not sufficient, the author may be able to provide a generic description of the source file so that a reader can convert the worksheet to the format of her choice.
For example, a simple data set (with variables and cases) should not be given in Excel format (which is proprietary), but should be given as a simple tab-separated text file. This standard format encodes all of the essential information of the data set, and can be used with any statistical software (including, of course, Excel). If an article has a more complicated spreadsheet file (with data and embedded formulas, for example), the author could still provide the file in the open document format (text-based, XML files, compressed into a ZIP file). An article with Mathematica worksheets could give HTML versions of the worksheets that a reader could convert to other computer algebra systems.
Here's a good way to think of this issue: the author should be free to use the tools of his choice in creating documents, but these should provided in open-source formats if possible, so that the reader can use the tools of her choice in processing the documents.