There are a number of techniques typically used when writing rendering tests; these are especially using for visual tests which need to be manually judged and following common patterns makes it easier to correctly tell if a given test passed or not.

Indicating success

Success is largely indicated by the color green; typically in one of two ways:

  • The green paragraph: arguably the simplest form of test, this typically consists of single line of text with a pass condition of, “This text should be green”. A variant of this is using the background instead, with a pass condition of, “This should have a green background”.

  • The green square: applicable to many block layout tests, the test renders a green square when it passes; these can mostly be written to match this reference. This green square is often rendered over a red square, such that when the test fails there is red visible on the page; this can even be done using text by using the Ahem font.

More occasionally, the entire canvas is rendered green, typically when testing parts of CSS that affect the entire page. Care has to be taken when writing tests like this that the test will not result in a single green paragraph if it fails. This is usually done by forcing the short descriptive paragraph to have a neutral color (e.g., white).

Sometimes instead of a green square, a white square is used to ensure any red is obvious. To ensure the stylesheet has loaded, it is recommended to make the pass condition paragraph green and require that in addition to there being no red on the page.

Indicating failure

In addition to having clearly defined characteristics when they pass, well designed tests should have some clear signs when they fail. It can sometimes be hard to make a test do something only when the test fails, because it is very hard to predict how user agents will fail! Furthermore, in a rather ironic twist, the best tests are those that catch the most unpredictable failures!

Having said that, here are the best ways to indicate failures:

  • Using the color red is probably the best way of highlighting failures. Tests should be designed so that if the rendering is a few pixels off some red is uncovered or otherwise rendered on the page.

  • Tests of the line-height, font-size and similar properties can sometimes be devised in such a way that a failure will result in the text overlapping.

  • Some properties lend themselves well to making “FAIL” render in the case of something going wrong, for example quotes and content.

Other Colors

Aside from green and red, other colors are generally used in specific ways:

  • Black is typically used for descriptive text,

  • Blue is frequently used as an obvious color for tests with complex pass conditions,

  • Fuchsia, yellow, teal, and orange are typically used when multiple colors are needed,

  • Dark gray is often used for descriptive lines, and

  • Silver or light gray is often used for irrelevant content, such as filler text.

Obviously, none of this can be taken as absolute rules, as testing of color-related functionality will necessitate using some of these colors!