This paper is concerned with fair and valid testing. It reports on a long-term project aimed at creating visual accessibility resources in mathematics computer-administered test items in a large-scale assessment system. We discuss our experience designing and developing pop-up illustration glossaries (IGs) for second language learners (SLLs) -- students who are not native users of the predominant language in the society in which they live, are typically schooled in that predominant language, and face the challenge of learning content and demonstrating knowledge in a language that is not their first language. An IG is a visual representation of a word/term identified as likely to be linguistically challenging to a substantial proportion of SLL test takers and which appears on the screen when the test taker clicks on that word/term. Because they interact with the user, IGs are more sensitive than other accessibility resources to students' individual needs -- a characteristic that addresses the linguistic heterogeneity of SLL populations. We discuss the extent to which IGs contribute to ensuring valid, fair assessment for SLL students, and the implications of the lessons learned for improving assessment systems.