Purpose and context determine whether specific text is decorative. According to the definition of “pure decoration” in the Understanding SC 1.4.9: Images of Text (No Exception) document in the Web Content Accessibility Guidelines, decorative text elements “can be rearranged or substituted without changing their purpose.” If you can change or re-order the text with no meaningful effect, this indicates the text provides no information and can be considered decorative.
For example, consider an online math lesson that has a background image of jumbled numbers and math symbols. If the numbers and symbols can be rearranged or swapped out without changing the image's purpose, the text functions as decorative.
This definition sets a fairly high standard for decorative text. In general, users with certain disabilities have a better experience when you avoid using images of text, and instead include text that users can interact with or modify as needed.
Learn more about other accessibility considerations in web content in the Accessibility Concepts for Marketing and Content Creators course from Accessible Web Academy.