The meta charset="UTF-8" tag should be placed within the head tag of an HTML document. It specifies the character encoding for the HTML document, ensuring that the browser correctly renders the text content.

The 'defer' attribute in a script tag ensures that the script is executed only after the HTML document has been fully parsed. This is useful for scripts that rely on the complete DOM being available before they run.

Media queries in CSS are used to apply different styles to a website depending on the characteristics of the user's device, particularly the screen size. This technique is essential for creating responsive web designs that provide an optimal viewing experience across various devices.