Plain text format contains no formatting information.  This is in contrast to rich text format includes formatting details such as font size, style, color, and other elements like images, tables, and links.  

Math Accessibility Guidelines

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Typed text is more legible for those with cognitive or visual difficulties including dyslexia, dysgraphia, low vision, color blindness, and more. Variations in handwriting style, skewed text, writing that is too small or too big, block vs. cursive writing, and more can make handwritten text and math difficult to decipher. As such, it is highly recommended to work with typed text and equations over handwritten math.