the Serif font in scratch is usually represented as your browser's default serif font. however, there is in fact a font file for it, but it usually never shows up as the word "serif" is interpreted by CSS as a "default" font. however, i found out that by letting the font be "Serif" (in quotes), you can show the "real" Serif font in scratch. this is what it looks like (i am maybe the first one ever to use it in a scratch project?)
you're free to use this as much as you like