Here we see an example of the much beloved Roman custom of the "folded chasubles" (folded up and attached with buttons).
On certain days throughout the year the deacon and subdeacon do not wear the dalmatic and tunicle, as they are signs of joy (cf. see vesting prayers).
This is done, for example, in collegiate churches, as with the Pantheon (a.k.a. Basilica Collegiata di S. Maria Ad Martyres).