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Text that needs emphasis or special formatting should be done in the semantic way, especially if the formatting contains meaning. This includes bolding, italicizing, and insertions or deletions.
For people that navigate a webpage using speech commands or speech input devices, they need to know the accessible name of a form input or button, so they can activate it. Screen readers, braille displays, and other assistive technology also relay the accessible name to people using them to navigate.
If the visible name of a button or form input doesn't match the accessible name, this can be confusing at best. At worst, the site is not usable for all users.
Text and interactive elements need to have high contrast so that your content is readable to all users. High contrast especially benefits site visitors that may have low vision or experience color blindness. It can also help mobile users still see content even if the screen is being affected by sun glare.
A user that navigates by keyboard, speech recognition, or other assistive technology needs a way to be able to navigate out of a piece of web content. If the user requires a mouse to get away from the content, this can block content from users or force them to refresh the page to escape.
Users with low vision or reading disabilities benefit from having text with increased line spacing, enough margin and/or padding, and fonts that are easier to read. For users that may need even more spacing, or a different font size or type, rendering the content so that it is customizable to the user's needs is even more important.