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The title element is necessary for users to understand what the page is about, what site they are on, and if the page changed. Meaningful titles can also help with search engine optimization and make content more discoverable.
Table headers tell users which data cells are related to that topic. This makes it easier to understand how table data is structured, especially if using assistive technology to navigate a table.
Labels tell the user what to put into a text input, a checkbox/radio button, or what will happen if a button is pressed. Labels also provide a benefit of putting the user in the field if they click on an actual label for a text input.
Landmarks are helpful for understanding a broad sense of the page layout, and can help assistive technology users easily navigate to different areas of a webpage. Landmarks include a header, main content, footer, and navigation, among others.
Content that flashes repeatedly can be harmful to people with photo-sensitivity or seizure disorders like epilepsy. Flashing content can lead to reactions like vomiting and nausea, and trigger seizures if the flashes are severe enough.