The simplified Button markup introduced in #21266 appears to require updates in the core themes so that the new buttons match the legacy button styling. In the following themes, if you add some new buttons (select a mix with outline styles), their styling doesn't match how the theme handles legacy buttons. After the fixes in #21642 and #21816, the legacy buttons are looking pretty good! Looks like it's now the new markup that needs addressing.

Some of the button styles below are closer than others (e.g. the difference might only be a lack of uppercase or underline, but for others like 2019, the difference is more pronounced). Note that in each of the screenshots under "New buttons", two of the buttons should be rendered with outline styles.

Twenty Nineteen

Legacy buttons	New buttons

Twenty Seventeen

Legacy buttons	New buttons

Twenty Sixteen

Legacy buttons	New buttons

Twenty Fifteen

Legacy buttons	New buttons

Twenty Fourteen

Legacy buttons	New buttons

Twenty Thirteen

Legacy buttons	New buttons

Twenty Twelve

Legacy buttons	New buttons

Twenty Eleven

Legacy buttons	New buttons

I haven't included themes 2010 and 2020 in this list because they're looking pretty good to me so far.

Is this the best place to log this issue for these themes, or should individual tickets be opened separately? (I wasn't sure if there might be individual tracking issues already)

CC: @youknowriad