The word is NOON.
SIS
this \
noon
SOS
SWIMS
The word SWIMS, when written in upper case letters, is the longest word in the English language that reads the same both forward and upside-down with no reading difficulty.
I only know that x and o stay the same
There is no upside down capital A symbols in runescape.
They are also letters upside down: y n u o s x and z Comment: Don't think that's it. Y is not an upside-down h. If this is it, c and l are missing from the set, and you'd have to allow p as an upside-down b. Besides, the answer could involve the capital letters instead.
They're OIXZH
NOON
I upside down is I and MOM upside down is WOW Some words which look the same whan written upside down are BOX, DID, HOOD and BOOB. Try creating other words using the key below, which lists letters and their upside down equivalents: Capital letters: B - B. C - C. D - D. E - E. H - H. I - I. M - W. O - O. W - M. X - X. lower case letters: b - p. c - c. d - q. i - i. l - l. m - w. n - u. o - o. p - b. q - d. u - n. w -m. x - x