Speakable web content is designed primarily to be read out loud by digital assistants like Google Assistant or Alexa. While many web crawlers can tell the difference between normal text on a web page and text that is supposed to be speakable, browsers currently do not display it differently. But with CSS styling you can give your readers a visual clue about which text is also speakable.
Most HTML markup is designed so that it web browsers will interpret it and display it in a way that is easy for viewers to understand. For example, text inside heading tags is often given a bigger font size than text inside paragraph tags. By default this is not the case with speakable markup.
In a previous post I showed three different ways to indicate what content is speakable in your HTML using the Speakable Specification. If you choose the method with CSS selectors, it's also easy to add some styles to make the speakable content visually different from other text.
So for example, using the example from the previous blog post with the following JSON+LD structured data: