BOSTON--Yesterday's election gave the Boston City Council its first progressive majority, as neophyte Rosario Salerno proved the experts wrong by winning an at-large council seat.

Early precinct results showed conservative incumbent Albert L. "Dapper" O'Neil as the top votegetter in the at-large race, with 47,817 votes. But supporters of rent control and stronger civil rights policies won three of the panel's four at-large seats. Christopher Ianella was re-elected with 45,472 votes. Rosario Salerno received 39,089 votes and incumbent Michael J. McCormack came in fourth with 36,326 votes. In the nine district elections every incumbent was reelected.

The most predictable of last night's winners was Ianella, who has gone undefeated since 1958. In this election, Ianella concentrated on the issues of affordable rental housing and basic changes in city ordinances.

But the 1987 race, which saw all the incumbents returning to their seats, proved surprising with the election of Salerno to the one open seat.

An ex-Benedictine sister, Salerno was the most unconventional candidate in the city councilor race and she won big citywide. She was particularly popular in Boston's Mission Hill section, the North End and in the Back Bay, coming in first in all three of these precincts.

Salerno's win in the North End as an Italian candidate is typical of Boston's ethinically-oriented politics. But, Bob Case a spokesman for the Salerno campaign said that her appeal is not ethnic but issue-oriented and unifying.

"There is a kind of old school politics in Boston. It is divisive and tries to use ethnic and religious backgrounds to overturn people," Case said.

The upset of the evening came when member of the Boston School Committee Joseph W. Casper failed to get elected to the Boston City Council. He finished in fifth place behind his most apparent adversary Salerno with 32,548 votes.

The Salerno-Casper escapade exploded last month when Casper accused Salerno of being a lesbian, an allegation which she denied. While his attack could have proven devastating, many seem to feel it actually gained Salerno support.

Her handling of Casper's alleagations against her was especially effective in creating a strong coalition of gay support in Boston.

Will Hutchinson, chairman of the Greater Boston Lesbian/ Gay Political Alliance said that 90 percent of Boston's large gay community voted for Salerno and that some were very active in her campaign.

Salerno, a chaplin at Boston College, has managed to create a citywide coalition of support that includes a cross-section of Boston's population.

The issue of childcare, political observers said, is symbolic of Salerno's broad-based campaign. She proposes to set-up an advisory committee citywide to study the issue of childcare along with a linkage-type program where businesses are required to set-up their own daycare centers.

"All women are interested in Rosario Salerno, professional women and low-income women who have to work. There just is not enough childcare available," said Karla Rideout, Program Director of the Jamaica Plain Neighborhood House in Boston.

Salerno topped the ticket in South Boston. Many attribute her success in the inner-city to her housing policy.

Aside from the at-large council candidates nine councilor were elected from city districts. They are: incumbent Robert Travaglini, incumbent James M. Kelley, incumbent James E. Byrne, incumbent Charles C. Yancey, incumbent Thomas Menino, incumbent Maura Hennigan Casey, incumbent Bruce C. Bolling, incumbent David C. Scondras '67, and incumbent Brian McLaughlin.