The character limit for alt text is 125 characters, which is short, but just do the best you can.
Keep your alt text fewer than 125 characters. Screen-reading tools typically stop reading alt text at this point, cutting off long-winded alt text at awkward moments when verbalizing this description for the visually impaired.
Terrill Thompson did create a test page for it and documented his results: No screen reader cut off text altogether. JAWS – and I have tested this with a more recent version of it as well – will split up the alternative text into multiple graphics with shorter alternative text.
Screen readers do not just hide the rest of the text.
Additionally, I am pretty sure that the cut-off mentioned in Firefox does not exist anymore. At least I did not come across any truncated alternative texts.
My best guess is that some people have observed JAWS’ behavior and assumed that the text was truncated where it was only split up.
Alternative text best practices
I helped write a whole tutorial on images, so I don’t want to go into the details here, but the general rules are:
- If an image meaning or shows something, it needs alternative text.
- Alternative text depends on the context and usage of the image.
- Describe the image as succinct as possible, but also as detailed as needed.
Writing alternative text is an art more than a science, and sometimes you’ll leave out essential information. My litmus test is “would I picture an approximation of the image when it was described to me over the phone using the alternative text.”
Note that many people use the phrase “alt text”, because the “traditional” way to use alternative text is through the
altattribute in HTML. These days, alternative text for images can come in different methods, including ARIA labels and descriptions or in some cases SVG titles. ↩