We currently only work with Chrome, Safari, and newer Internet Explorer versions. For early stage software companies, there's extra development to accommodate all browsers and versions. However, our product is completely mobile friendly.
Only a small minority of internet users are using the Internet Explorer version this customer has (reference). It's standard practice for software companies to hold off on accommodating all browsers/versions (and it actually is very resource intensive to support all these!). Our co-founders are happy to share more insight around this, and you can reach out to email@example.com for more information.