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Some users need to increase the font size or zoom in on content in order to access content. If zooming is disabled, the text could be harder to see, or require additional assistive technology like magnifiers to access the content. Some pages may also not support responsive design out of the box, and may need zoom to even use content on mobile.
Picking out links from a block of text can be challenging for users with low vision or color vision disabilities, if color is the only way to tell the difference. Providing additional cues like underlines or backgrounds can make it easier to find links without needing color alone.
People that don't use a mouse, use assistive tech like speech recognition or on-screen keyboards, and take longer to read a page may need more time to complete tasks or navigate a page. Setting time limits on content, or not letting users know there is a time limit, means some content won't be detectable or reachable by your users.
User interface (UI) elements like links and buttons are critical in telling a user what will happen if the element is selected. If the element just uses an icon as the link without a text alternative, someone using assistive tech may not know where they will end up when clicking it, or what command to give the computer to make the feature work.
Some icons may not also be as intuitive to users from different cultures, or who aren't visually-oriented learners.
If your form has a series of checkboxes or radio buttons, like a "yes/maybe/no" set of options, it helps users to understand what overall question each of those buttons is answering. This grouping can tie inputs that relate to each other as well, instead of seeming like random inputs.