|Image from end of death register book,|
Allegheny City, Pennsylvania, 1891
If you click on "Browse all published collections" on the FamilySearch site, you won't see a record set for Allegheny City deaths. But FamilySearch actually does have these records digitized and available to browse in its collection called "Registrations of deaths in the city of Allegheny, 1876-1907." Each listed record group with a picture of a camera under "Format" means it can be viewed online. (A reel means you have to order the microfilm for viewing at your local Family History Center.)
Glancing at the thumbnail images will give you a sense of where the handwritten surname index starts for each volume; it's usually somewhere in the middle and not at the beginning or end due to the way the book scans were saved. And sometimes there's a second index further down in the same file where the next book of death records begins. You can use these indexes to locate the page number of the death record.
My Uncle Emil did indeed die on May 4, 1903, due to "abcess [sic] of liver." The record also states that he had resided at 707 Middle Street in the 3rd Ward for 30 years and that he was buried in St. Peter's Cemetery on May 7th (although a photo of his headstone on FindAGrave indicates that he's in Highwood Cemetery).
By browsing for other relatives, I found cousin August Huber who died in 1882 at 7 weeks of age due to eclampsia, as well as another cousin's father-in-law who died in 1903 of "mania a potu" or madness from drinking. I had their dates of death from other sources (church record for the first and a newspaper obituary for the second), but now I have more details, including where they are buried.
Be sure to search the FamilySearch Catalog for your ancestors' towns to see if digitized images are available. As this example shows, some online records don't appear on the published collections list.
Note: "Registration of births in the city of Allegheny, 1878-1907" is also available on FamilySearch, although five years are missing.