About wrong display of some symbols in html

In the first section in html, with some browsers, the following symbols
that should appear as calligraphic A,B,C,R, are not properly displayed as such, but appear as boxes.

Where I could check, the display appeared correct with:
In Windows systems, the symbols were not recognized (appeared as boxes) with

And they appeared under Firefox in the right form but with wrong height (taking a huge vertical space that appears as big blanks between lines).

(I did not check IE 9 ; if you observed something else with a browser I did not mention here, you can please inform me so that I will add the notice)

If you have not a browser giving a right display, you can still use the pdf instead (where the display is always correct).
(There would be possible variants of calligraphic letters to use : A, B, C, R, or 𝒜, ℬ, 𝒞, ℛ or 𝔅, ℭ, ℜ, but, well, should I make copies of the same texts with different fonts ?)

About some 404 errors

The logs of this site indicate that there are some requests to wrong urls (but do not show if they are from browsers used by humans or from robots), especially to settheory.net/world/ and other urls ending with /. Where do these final / come from (can you explain if you had this problem) ? The correct url is settheory.net/world (or if you wish, /world.htm)