In Word 2013, under File / Options / Advanced / Image Size and Quality, you can check the box to tell Word "Do Not Compress Images in File" and you can also change the default output to 220 ppi.

I have no idea whether this would affect saving a Word document as a web page or saving equations as graphics ... but you could play with it and see if it helps

The Save As dialog box Tools / Web Options also allows you to choose a screen size and pixels per inch for the monitor you plan to view the web page on. Presumably graphics would be saved in higher quality if you choose 1920x1200 and 120 ppi