I want to know the answer to the following question:
What is common about the surviving mosaics from the 7th-9th centuries?
All the background I'm provided with is that "Among the rare examples are the 6th-century Christ in majesty (or Ezekiel's Vision) mosaic in the apse of the Church of Hosios David in Thessaloniki that was hidden behind mortar during those dangerous times.". Is that enough to answer the question?