Huddersfield, who needed a point to be sure of avoiding relegation, took a shock lead in the 50th minute at Stamford Bridge on Wednesday.
Cesar Azpilicueta drilled over a low cross that prompted a panicked clearance from Mathias Zanka and the ball hit Alonso in the face before rebounding into the Huddersfield net.
A goal-mouth scramble in the final minutes saw the loose ball fall to Andreas Christensen, whose header was touched onto the post by Lossl.
In the end, Town sealed another top-flight campaign with a game to spare, meaning a party atmosphere will be most certainly present when they bring down the curtain on a stunning season at home to Arsenal on Sunday.More news: Kilauea volcano eruption: After lava, Hawaiians brace for acid rain and vog
Huddersfield's valiant defensive show sparked jubilant scenes on the remaining whistle, whereas Chelsea midfielder Cesc Fabregas appeared to snub a handshake from Wagner within the moments after the sport.
"We have had many, many chances to score in games - to win games that we didn't win, games that we drew", he told Sky Sports. Their deep and compact defence eventually took the sting out of Chelsea's early urgency, and Lossl took his time with every goal kick.
In fact, Huddersfield have drawn a blank in 13 Premier League away trips this season - a joint-record (also Norwich City 1994-95, Southampton 1998-99 and Middlesbrough 2002-03). We were everything but a Premier League club last summer but we have given it a go.
Antonio Conte stood by his team selection after Chelsea's 1-1 draw at home to Huddersfield.
The Blues were 1-0 winners against Jurgen Klopp's side last weekend to leave them still fighting for a Champions League qualification spot next term.
The result meant Chelsea's already faint hopes of snatching a Champions League place faded further. "We had great performances individually but it's been all about the team".