Pope John XXIII
Maintaining continuity with his predecessors, John XXIII continued the gradual reform of the Roman liturgy, and published changes that resulted in the 1962 Roman Missal, the last typical edition containing the Tridentine Mass established in 1570 by Pope Pius V at the request of the Council of Trent and whose continued use Pope Benedict XVI authorized in 2007, under the conditions indicated in his motu proprio Summorum Pontificum. In response to the directives of the Second Vatican Council, later editions of the Roman Missal present the 1970 form of the Roman Rite.

Q: John XXIII continued the gradual reform of what?
the Roman liturgy