Exactly what it says on the tin. All 26 letters, both upper and lower case, along with ALL punctuation, even though much of it is rarely used. On Firefox, no more emojis ruining the professional look of your Totally Legitimate Font! As for Chrome, certain hands and faces are seemingly unavoidable. Please see below.
Unfortunately, many lower case characters seem to be unable to be displayed. For best results, as well as Gaster's text, use ALL CAPS.
— Problem characters include: s, p, o, ¢, and £, which have no unicode equivalents and can only be displayed as boxes, and ÿ, which is the windows logo, and is the only character not included.
— In addition, on certain browsers, such as Chrome, the characters ?, A, G, I, and K will display as emojis. As mentioned above, this is unavoidable. Many more characters will also not display at all, including s, p, o, ¢, and £, as well as ¤ and ¥. Apologies.