No goal controversies. No straight red cards. No players seriously injured by bad tackles.

The absence of contentious incidents involving referees at the European Championship has delighted UEFA.

"We are happy with the performances of all these referees," UEFA's head of refereeing, Pierluigi Collina, said Friday about the 18 teams of match officials, who now receive expert tactical advice ahead of each assignment.

Twelve referees and their regular teams of assistants have been retained for 15 matches in the knockout rounds that start Saturday.

Unlike at previous tournaments, no match officials leaving after a group stage now extended to 36 games will go because he made a game-changing error.

Four years ago, the failure to give a goal for Ukraine against England — in an attacking move that began with an offside decision also missed — meant the Hungarian team of match officials made an early exit from Euro 2012.

Collina, seen by many as the best referee ever with his trademark bald head, singled out his satisfaction that all offside decisions at Euro 2016 have been correct when a goal — either given or disallowed — was at stake. The best of those rulings was Jamie Vardy's goal in a 2-1 win for England over Wales when a defender's header teed up his close-range shot.

"It has been a very brave decision, a very good decision with good teamwork," Collina said in praising the officiating team led by German referee Felix Brych.

A referee's tactical briefing before each match is an innovation for this tournament.

Collina said match officials now get a one-hour briefing from expert coaches who analyze video of both teams.

"It is working very well," the Italian former top referee said. "These are things we want our referees to know, tactics. If you are surprised by something, it is very probable you are incorrect. They must know, before, what is going to happen."

Collina and his UEFA referee advisers also tried to educate coaches and players before the first kick off by briefing each of the 24 teams at their training camps.

"We see that players and managers understand, particularly this year with changes on the law in offside implemented from the start of this tournament," he said.

Collina suggested this better understanding can explain the drop in numbers of red cards and violent tackles.

After three players were sent off in the 31-match program at Euro 2012, just two red cards have been shown in 36 matches in France. Both players dismissed — Lorik Cana of Albania and Austria's Aleksandar Dragovic — got second yellow cards.

"There is no need" for instant red cards, Collina said. "We tried to convince them. We want respect on the field of play, we want players playing in this competition, not recovering from a brutal tackle. I think it has been understood."

Despite the sight of ripped shirts for Switzerland players against France, Collina dismissed a suggestion that man-marking duties were too physical at free kicks and corners.

"Grabbing shirts in the penalty area cannot be considered a trend," he said.

Collina revealed his suggestion to referees at their mid-tournament briefing on Friday: Think like Muhammad Ali.

"They must float as a butterfly when not needed," Collina said, adding that a referee is best when not being noticed. "When he is needed, he must sting like a bee."

The list of retained Euro 2016 referees:

Martin Atkinson (England), Felix Brych (Germany), Cuneyt Cakir (Turkey), Mark Clattenburg (England), Jonas Eriksson (Sweden), Viktor Kassai (Hungary), Bjorn Kuipers (Netherlands), Szymon Marciniak (Poland), Milorad Mazic (Serbia), Nicola Rizzoli (Italy), Damir Skomina (Slovenia), Carlos Velasco (Spain).