CSS単位別小数点以下の扱い

Percentage

font-size: 1.5545555555% (1.55556%)

Font-relative lengths

font-size: 1.5555555555em (1.55556em)
font-size: 1.5555555555ex (1.55556ex)
font-size: 1.5555555555ch (1.55556ch)
font-size: 1.5555555555rem (1.55556rem)

Viewport-percentage lengths

font-size: 1.5555555555vw (1.55556vw)
font-size: 1.5555555555vh (1.55556vh)
font-size: 1.5555555555vmin (1.55556vmin)
font-size: 1.5555555555vmax (1.55556vmax)

Absolute lengths

font-size: 1.5555555555cm (1.55556cm)
font-size: 1.5555555555mm (1.55556mm)
font-size: 1.5555555555in (1.55556in)
font-size: 1.5555555555px (1.55556px)
font-size: 1.5555555555pt (1.55556pt)
font-size: 1.5555555555pc (1.55556pc)

Angles

transform: rotate(1.5555555555deg) (rotate(1.55556deg))
transform: rotate(1.5555555555grad) (rotate(1.55556grad))
transform: rotate(1.5555555555rad) (rotate(1.55556rad))
transform: rotate(1.5555555555turn) (rotate(1.55556turn))

Times

animation-delay: 1.5555555555s; (1.55556s)
animation-delay: 1.5555555555ms; (1.55556ms)

おまけ (Firefox は桁数で判断してるっぽいので)

font-size: 1.5555555555px (1.55556px)
font-size: 10.5555555555px (10.5556px)
font-size: 100.5555555555px (100.556px)
font-size: 1000.5555555555px (1000.56px)
font-size: 10000.5555555555px (10000.6px)
font-size: 100000.5555555555px (100001px)
font-size: 1000000.5555555555px (1e+06px)