The user highlights a block of text. They select the 'Speak' tool with a key combination or from a right click menu. The text is spoken. Could have a few different voices, American, British, etc.
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For now opening some webpage don't show the Read mode button. Maybe there's a way to read highlighted text with an extension, I searched for read text and found many results but don't know if they allow to read the highlighted text only rather that the whole page.