What is the function of the 'viewport' meta tag in responsive web design?

The 'viewport' meta tag plays a crucial role in responsive web design by controlling the layout on mobile browsers. This tag allows developers to set the visible area of the webpage according to the device's screen size. By defining parameters such as width and scale, it enables a webpage to render correctly on different devices, ensuring that users have an optimal viewing experience.

For example, including <meta name="viewport" content="width=device-width, initial-scale=1.0"> instructs the browser to match the screen's width and sets the initial zoom level. This prevents sites that are designed for larger screens from displaying inappropriately on smaller screens, which can lead to horizontal scrolling and a poor user experience.

In contrast, the other options relate to different aspects of web development. Specifying the character set pertains to how text is encoded and displayed, while optimizing images for different screens involves techniques such as responsive images or media queries but is not directly managed by the viewport meta tag itself. Setting default styles for all browsers usually involves CSS reset stylesheets or normalization techniques, rather than the viewport meta tag.