Kaoru Mori has a bit of a Victorian England obsession; she has stacks of pictures and information and references on the period, and it shows in her work. Unfortunately, most of it is easily lost on the rest of us.
To help out, Mori has also published the Emma Victorian Guide, a companion reference to the series. And for those of us not fluent in Japanese, the IIChan Translation Group does their part with "Whisper's Guide to Emma's World." It's a short feature after every episode, and two pages after every volume, explaining references.
For easy searching, I've pulled out the manga-page extras and listed the tidbits explained on each; the anime follows the first part of the manga closely, so the historical notes are the same.
See Crumpets and Corsets for more historical references.
Volume 1 notes 1
The Crystal Palace/Great Exhibition
Mudie's Library
Coming Out balls
Board school
Volume 1 notes 2
The new aristocracy
Fox hunting
The mail
Maids (general background)
Lighting
Volume 2 notes 1
Morning calls
Harrods Department Store
Eton College
Dinner party
Volume 2 notes 2
Card games
Maids (lady's maid, parlormaid)
Flower girls, orange girls
Midland and Great Northern Railroads
Hansom Safety Cab
Volume 3 notes 1
The Industrial Revolution
Incomes
Arthur Schopenhaur
Cooks
Volume 3 notes 2
Dances (quadrille, polka, waltz)
Banbury Cross
Servants' dress
New money vs. old money
Japonaiserie art
Volume 4 notes 1
The Derby
Gretna Green (elopements thereto)
The Barber of Seville
Volume 4 notes 2
Debrett's and Burke's (directories)
Rotten Row
Big Ben
Corsets
Volume 5 notes 1
First names
Chaperones
London's atmosphere
"Mrs. Trollope"
Volume 5 notes 2
Hair decorations
Hats
The guinea
Operas