What are the functions of the following HTML tags: h1, p, b, i, br, and what will happen if I change h1 to h3?

Here are the answers to your questions:

a) The title of the web page is defined within the <title> tags.
The title of the web page is:

$$\boxed{\text{Supermarket}}$$

b) The <body> and </body> tags define the main content of the HTML document. All the visible content of a web page, such as text, images, links, lists, and tables, is placed within these tags.

c) When you change the <h1> and </h1> tags to <h3> and </h3>, save the file, and refresh the browser, you will observe the following:
The text "Supermarket" will appear smaller and less prominent.
The reason for this change is that <h1> represents the most important heading, which is typically displayed in the largest font size by default. <h3> represents a third-level heading, which is displayed in a smaller font size than <h1>. HTML provides six levels of headings, from <h1> (largest) to <h6> (smallest).

d) The functions of the following tags are:

• <h1>: Defines the most important heading on a web page.
• <p>: Defines a paragraph of text.
• <b>: Defines bold text.
• <i>: Defines italic text.
• <br>: Inserts a single line break.