Store user data for more than 20 hours. Provide a warning of the duration of user inactivity at the start of a process. Back to top Guideline 2.3 – Seizures and Physical Reactions Do not design content in a way that is known to cause seizures or physical reactions. 2.3.1 Three Flashes or Below Threshold Level A Web pages do not contain anything that flashes more than three times in any one second period, or the flash is below the general flash and red flash thresholds. Note 1: Since any content that does not meet this success criterion can interfere with a user's ability to use the whole page, all content on the Web page (whether it is used to meet other success criteria or not) must meet this success criterion.