Several Web animation methods were independently assessed on fast and slow systems running two popular Web browsers under MacOS and Windows. The methods assessed included those requiring programming (Authorware, Java, Javascript/Jscript), browser extensions (Flash and Authorware), or neither (animated GIF). The number of raster scans that an image in an animation was presented for was counted. This was used as an estimate of the minimum presentation time for the image when the software was set to update the animation as quickly as possible. In a second condition, the image was set to be displayed for 100 msec, and differences between observed and expected presentations were used to assess accuracy. In general, all the methods except Java deteriorated as a function of the speed of the computer system, with the poorest temporal resolutions and greatest variability occurring on slower systems. For some animation methods, poor performance was dependent on browser, operating system, system speed, or combinations of these.