Very interesting project!
In a perfect world, the right form of |s| would be automatically handled via OpenType features in the font, but support in applications is still spotty (https://www.typotheque.com/fonts/opentype_feature_support), more so in browsers (http://caniuse.com/#feat=font-feature).

I’ve put an OpenType feature which follows the 18th century English language convention for long s, in several of my typefaces—but none of them are revivals from that era.

sub longs' f by s;
sub longs longs' by s;
sub f longs' by s;
ignore sub longs' @hist1;
sub longs' by s;

(@hist1 = lower case characters)

@Nick,

I'm trying to understand your code. Don't you also need to have a "sub s by longs;" at the beginning to perform the majority of subsitutions? And, the last line seems to revert all substitutions? How to you express that an "s" at the end of a word should remain "s"?

@riccard0

I can see two reasons against encoding this in the font files. First, is it acceptable/legal for users like me to edit the font files if a font doesn't have the feature I need? Second, I'm not sure that the rules for handling longs are set -- they may vary from language to language or era to era. Having several features in every font file may be an option, but using the human-readable (albeit with unicode code point) 'text-transform' property seems like a more compact and editable solution?

First, is it acceptable/legal for users like me to edit the font files if a font doesn't have the feature I need?

Not unless the license allows it, but you can select an appropriate font and serve it as webfont.

the rules for handling longs [...] may vary from language to language or era to era

They do. That's why you have both language and historical related features.
As I said, that would be the right solution for the user in a perfect word, where copying the text and pasting it using a different font would not end up with characters either missing or rendered with yet another different font.
Today, especially for the creator, your solution is probably the best, and indubitably a very clever one.

What Nick used is Contextual substitutions i.e. substitutions that occur only in a specified context (for example substitute an s only when followed by a letter). LuaTeX allows using .fea (Adobe feature) files to do that so as to get the functionality without modifying the font.

My AlQuds Arabic font featured a nicely extended letter /ra but it clashed with the following mark in a couple of specific cases. To correct it I used a contextual Marks feature to change the mark position when that combination occurs, implemented as an OpenType feature in MS VOLT tool. Iffue solved.

@howcome: The "ignore sub" takes care of that. It details when a substitution is not to happen. The net effect is that 'longs' is substituted by 's' unless it is followed by a lower-case character.

@howcome: true, there is no standard for long s usage.
I based my code on studying this excellent resource:
http://babelstone.blogspot.ca/2006/06/rules-for-long-s.html

Thanks to all for enlightening me. I've now also found an inconsistency in the original 1798 edition: "necessity" is spelled with double ss, on page 35; however, on page 42 it is written "neceſsity". So the practice must have caused some confusion, even back then.

Either that, or the setter had run out of 'ſ'.

For some reason, the links in the original posting have been removed. Here's another attempt:

http://css4.pub/2015/malthus/essay.html
http://css4.pub/2015/malthus/essay.pdf
http://archive.org/details/essayonprincipl00malt
http://www.wiumlie.no/2014/ibsen/digte.html#english