According to ECMA-48¹ 8.3.117, an attribute value of 21 is "doubly
underlined", while 22 is "normal colour or normal intensity (neither
bold nor faint)".
Additionally, 25 is "steady (not blinking)", which likely means neither
slow blink nor fast blink.
¹: http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-048.pdf
Signed-off-by: Roberto E. Vargas Caballero <[email protected]>