truespeed attribute | trueSpeed property
Default. Marquee computes movement based on 60-millisecond ticks of the clock. This means every scrollDelay value under 60 is ignored, and the marquee advances the amount of scrollAmount each 60 milliseconds. For example, if scrollDelay is 6 and scrollAmount is 10, the marquee advances 10 pixels every 60 milliseconds.
Marquee advances the pixel value of scrollAmount by the number of milliseconds set for scrollDelay. For example, the marquee would advance 10 pixels for every 6 milliseconds if scrollDelay is 6, scrollAmount is 10, and the value of trueSpeed is true.
The trueSpeed property indicates that the exact scrollDelay value specified is used to move the marquee text. If trueSpeed is false, all scrollDelay values of 59 or less are rounded up to 60 milliseconds.