Social media platforms are integral to public and private discourse, but are becoming less accessible to people with vision impairments due to an increase in user-posted images. Some platforms (i.e. Twitter) let users add image descriptions (alternative text), but only 0.1% of images include these. To address this accessibility barrier, we created Twitter A11y, a browser extension to add alternative text on Twitter using six methods. For example, screenshots of text are common, so we detect textual images, and create alternative text using optical character recognition. Twitter A11y also leverages services to automatically generate alternative text or reuse them from across the web. We compare the coverage and quality of Twitter A11y's six alt-text strategies by evaluating the timelines of 50 self-identified blind Twitter users. We find that Twitter A11y increases alt-text coverage from 7.6% to 78.5%, before crowdsourcing descriptions for the remaining images. We estimate that 57.5% of returned descriptions are high-quality. We then report on the experiences of 10 participants with visual impairments using the tool during a week-long deployment. Twitter A11y increases access to social media platforms for people with visual impairments by providing high-quality automatic descriptions for user-posted images.
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