Recently the Yahoo App (Android version) is not responsive any more, and there seems to be ways to fix it by duplicating CSS styles to the email body with other restrictions, but it adds quite a bit work. If we need to update something, we need to update two sets of CSS.

Based on the Litmus's study about the email client market share, Yahoo mail is 2%, Yahoo App might be much less. https://litmus.com/blog/the-2017-email-client-market-share-infographic?utm_campaign=newsletter&utm_source=email&utm_medium=marketing&utm_content=free

So how important is it to spend a lot extra time to solve one single non-critical issue in an email client that has a small market share, such as not being responsive in Yahoo App (Android version)? Do you test your email with all email clients available on Litmus or select some of them?