This example is a test of reaction-time, where a button appears after a random period of time, and the user is asked to click it as soon as it does. But it takes more time with the keyboard to reach and actuate a button that's just appeared, than it does with the mouse; and it takes much longer to hear an element described in spoken text, than to visually read it.
So even though the test's component (the button) can be presented as text, its purpose is to test reaction time, and that purpose can't be represented as text. Therefore, it must have additional descriptive text to explain the situation to users.
A button will appear below this text within the next 20 seconds. You should click the button as soon it appears, to test your reaction time:
Please note: this test is only effective for sighted mouse users. If you're using a screenreader or braille-reader, or navigating only with the keyboard, this will not be an effective test of reaction time.
The JavaScript code for this example can be found in: JS_G_A3.js
The CSS for this example can be found in: JS_G_A3.css