I would really hesitate to say that this special election is a signal of the beginning or end of any trend at this point. It was a low turnout, off-year race that many people didn't even know was happening. Republicans in Jax have historically done well in these races.
The governors race in November will be the best indicator of where the city might be heading politically.
Quite so. Jacksonville is a purple city that's increasingly favorable for Democrats due to population trends and the "Great Sorting" of traditional Republican families self-segregating in the bedroom counties. However, Republicans have a lot of built in advantages in funding, party infrastructure and bench of candidates. While I don't expect far right Republicans or Trumpers to do well outside bright red districts for the foreseeable future, non-fringe Republicans will be able to win elections here for a long time.
Polson did incredibly well for a Democrat in this race - previously, the only Democrats to win countywide local races were people like Hazouri and Crescimbeni who had name recognition and status from before the city shifted Republican in the 1990s. Polson came within a few thousand votes of winning. This is a trend up from previous non-Tommy races such as the Clerk of the Court, where Jimmy Midyette came within 12k votes of Jody Phillips despite Phillips having all the advantages.
Howland locked up his donor class and solid Republican voter base early, and (in my opinion) he came across as a good speaker and generally upbeat, likeable guy. That goes a lot further than people sometimes give it credit for. He's also riding the backlash to national Dems which swings every time the White House changes parties. Plus he successfully dodged or threaded the needle on a number of controversial issues that could have put people off him or energized the Dems, including Confederate monuments, school funding, election fraud, Curry, and Trump, and his "defund the police" lie really resonated with his base and moderates. On the other hand, winning for him took a metric shit ton of money, including tens of thousands of dollars from Tallahassee and a $50,000 donation from the local party, which I don't believe they've ever done before.
As an off cycle election, it was more about turnout than a gauge of where the city really is. I believe we'll see a few more cycles where Duval goes blue in state and national elections while Republicans maintain power in local elections. I do think we'll start seeing more success from true moderate Republicans like Matt and Joe Carlucci and Randi DeFoor who can successfully pull in NPAs and Dems.