What does it do?
Checks whether a website defines text alternatives for purely visual elements, so called alternative text. For example, pictures, audio or video should have a text equivalent.
Why is it important?
Not including alternative text means that search engines and users with visual impairments are unable to identify that content. This is particularly important where an image contains text that is unavailable elsewhere, such as a phone number or company name.
Including alternative text is one of the most crucial first steps towards making a website accessible, optimised for search engines and compliant. It is also remarkably easy to address. All visual elements of a page should have a text equivalent, for the benefit of those who cannot see the image (e.g. visually impaired or blind), and for search engines, which cannot understand images like people can.
How is it measured?
The use of alternative text is measured for every element of every webpage that could use it (typically images, audio and video). The score is taken from the proportion of alternative text that is specified for each of those elements, weighted for the relative importance of the pages they appear on. The best possible score would come from using alternative text everywhere that could use it.
Images with a width or height of one pixel are excluded automatically – i.e. they do not need alternative text to be specified. Almost exclusively these images are used for tracking purposes, e.g. Analytics ‘beacon’ image, or as layout hacks, and they have no impact on accessibility or SEO.
Links that contain only an image – but have no alternative text – are penalised far more than standalone images. This images are essentially blank links – neither search engines nor visually impaired users can tell what they point to.
Note that just including anything for alternative text defies the purpose of this test. Where identified, particularly bad or useless Alternative text is penalised. For example, alternative text such as “Image01.jpg” does not describe an image to a blind person or search engine.
This test is looking for the alt or longdesc attributes on visual elements such as the <img> tag.
An empty alt attribute is acceptable, but raises a warning (no score penalty is applied). Additional rules test for weak alternative text, such as:
- image.jpg (a filename)
- Click here (assumes use of a mouse, doesn’t provide a description)
- Image or Picture (no actual description)
Some images are automatically excluded, where they are recognised as belonging to an Analytics tool such as WebTrends. All images with a width or height of one pixel are excluded automatically.
A website may use images which it could argue need no alternative text, for example spacer images. The correct practice to use with these images is to specify an empty
alt value, e.g.:
<img src="image.png" alt="" />
Most images of this nature are more properly found defined in CSS, where alternative text does not apply.
How to improve this score
Wherever possible, specify useful alternative text that describes the visual content. This is a very simple process, and is supported by virtually all Content Management Systems and website editors.
How to use this test effectively
If this score is low, it should be one of the highest priorities for improving search engine placement and accessibility. Adding useful alternative text is not technically difficult or time consuming.
Otherwise, use this test to ensure that alternative text is being applied and to a sufficient standard.
Because this test is automated, it may not appreciate special context applied to images in a site. For example, it is conceivable that the most appropriate alternative text for a given image is “Image”, for some special cases. These instances are generally extremely rare, and great care has been taken to avoid false positives of this nature.
By extension, just because apparently valid alternative text has been found, does not mean that it is useful for end-users and search engines. You should manually review your alternative text using Sitebeam where possible, in the context of your own site.