I hate to be this guy… But Microsoft does not even view Internet Explorer as a browser anymore. They’ve urged people for numerous reasons (web standards, no longer actively updated, and security/hacking issues exploits) to move to more modern browsers.
The long of this answer is that Webflow is providing us tools to build beautiful sites because beautiful design/development is partially being brought to us by the new possibility that the advances in the web standards that new browsers support. Things like FlexBox and Grid, Interactions, and things like that are not guaranteed to work in legacy software.
I’d explain to your client that Internet Explorer has not been updated for years and will not display many websites properly at this point…
Not to sound heavy handed… But it’s no longer a browser that Microsoft or the web community recommend for more reasons than I have time to list in this response.