As of 1.13-pre6, the vanilla game files no longer contain the old font file ascii.png, but use vector-based tff font files for the unicode characters instead. If resource packs still contain ascii.png though, the game will display chaotic font gibberish (see screenshot). Further information on how to apply the ttf files to a resource pack or problems with the SGA font (ascii_sga.png), see MC-132707.

Note:

I tried to find a way to properly convert my old ascii.png into a proper ttf file, tested different font editors and even played around with editing the vanilla seven.tff. It turned out to be really hard to even find a good (and free!) font editor or to create a font where the letter spacing is properly displayed in-game (really hard to get right), or don’t get blurry letters (every deviation from the default 7x5 pixel font blurs the lines and edges of the letters >>> MC-132736) … Minecraft really has some bad vector scaling.

 In its current implementation, making new fonts appears to be a real pain in the butt. To fix this issue, it might be a good idea to still allow resource packs to optionally use the ascii.png instead of seven.ttf, or otherwise provide a tool to convert such png files into a ttf with proper letter spacing and scaling. Creating a new font for a resource pack should be a managable task even for less experienced resource pack creators.