When one wishes to refer to a specific decade (e.g. "The 1920s were a great time for indoor badminton."), one does not insert an apostrophe between the ultimate zero and the letter S because this is a barbarism. Then why, one wonders, does the auto-alert-to-correct on the board underscore 1920s in red when written without the extraneous unnecessary apostrophe?

It doesn't do that for me - I can write 1920s with impunity. It is probably a setting in your browser (or a grammar-checking add-on, if you have one).

Yes, I don't get that problem in Word, but I do seem to get it with the browser spellcheck. I find that different spellcheckers have different words which they seem to accept or not accept. That's why it's so important to check everything by eye as well.

The people who govern those spellcheckers and grammar-checkers might do well to consult the common authorities, e.g.

The Chicago Manual of Style, fifteenth edition (2003):

9.37 Decades. Decades are either spelled out (as long as the century is clear) and lowercased or expressed in numerals. No apostrophe appears between the year and the s.

the nineties

the 1980s and 1990s (or, less formally, the 1980s and '90s)

The Associated Press Stylebook (2002) adopts a rule very similar to Chicago's:

decades Use Arabic figures to indicate decades of history. Use an apostrophe to indicate numerals that are left out; show plural by adding the letter s: the 1890s, the '90s, the Gay '90s, the 1920s, the mid-1930s.

While the apostrophe is incorrect, it is becoming accepted in some places. Therefore, our policy here is that individual editors can change it if they prefer, but that we will not reject a quiz for its use.

English is a living language, and one with several main branches. I'm willing to live with a little imprecision.