Sass has deprecated 'mixed declarations'

[frankieroberto]

We’re now getting a number of deprecation warnings from the latest version of sass when used with NHS frontend.

Most of these are coming from the sass-mq library which we use.

GOV.UK Frontend is having the same issue (as our use of sass-mq was largely inherited from them), and have opened an issue to track this: alphagov/govuk-frontend#5143

In the short term the warnings can either be ignored or silenced.

Longer-term we might be able to remove the sass-mq dependency, as suggested by the GOVUK team:

It has been suggested that we can likely remove this dependency, as we primarily used it for ease of supporting legacy Internet Explorer versions which we stopped supporting with GOV.UK Frontend v5.