Calendar invitation email format viewed in gmail has problems

Calendar invitations sent by Zoho appear to be formatted in such a way that responses will generally not work when the email is viewed in the gmail web browser. This may be Google's problem due to linking of invitation receipts with their calendar app, I don't know if it's made worse by Zoho formatting or not. A screen shot of the email is attached.

In the invitation email there appear to be two separate fields where the invitee can respond yes, maybe, or no. However, the first field does nothing other than toggle the button color, while the second works properly. Most people on receiving an invitation will hit the button on the first field and assume it has sent a response.

When viewed with email clients such as Thunderbird the formatting works properly. I've been having issues with replies to calendar invitations, and this may be part of the problem. If it can't be fixed by Zoho this bug report should be closed.