The <br> tag in HTML stands for "line break." It is used to insert a line break within text, forcing the content following the <br> tag to start on a new line. This tag is an empty (self-closing) element, meaning it does not have a closing tag.

Syntax:

<p>This is some text.<br>This is on a new line.</p>

Key Characteristics:

1. Line Break: The primary function of the <br> tag is to create a line break within text, which is useful for formatting content where a new line is needed without starting a new paragraph.

2. Empty Tag: As a self-closing tag, <br> does not require a closing tag. It is written as <br> in HTML4 and <br /> in XHTML and HTML5.

3. Text Wrapping: The <br> tag affects only the text immediately before and after it. It does not create extra space beyond the line break, unlike block-level elements like paragraphs or headings.

Example:

<p>Line one.<br>Line two.<br>Line three.</p>

In this example:

• The text "Line two" appears on a new line below "Line one."
• The text "Line three" appears on a new line below "Line two."

Use Cases:

1. Addresses: Useful for formatting addresses where line breaks are needed between lines of the address.

   Example:

   <address>
     123 Main St.<br>
     Suite 4B<br>
     Springfield, IL 62704<br>
     USA
   </address>
   

2. Poetry or Song Lyrics: Ideal for displaying lines of poetry or song lyrics where line breaks are part of the formatting.

   Example:

   <p>
     Roses are red,<br>
     Violets are blue,<br>
     Sugar is sweet,<br>
     And so are you.
   </p>
   

3. Forms: Can be used in forms to create line breaks between input fields or labels without additional spacing.

   Example:

   <label for="name">Name:</label><br>
   <input type="text" id="name" name="name"><br>
   <label for="email">Email:</label><br>
   <input type="email" id="email" name="email">
   

Limitations:

1. Accessibility: Overuse of <br> tags for layout purposes can make the document less accessible, as screen readers may not interpret them well. It’s generally better to use CSS for layout and spacing.

2. Semantics: <br> tags are best used for inline formatting needs rather than structural changes. For more significant content separation, consider using block-level elements like <p>, <div>, or <section>.

Modern Alternatives:

For layout and spacing, CSS is usually a better approach than using <br> tags. CSS provides more control over spacing, alignment, and overall design, allowing for more flexible and accessible layouts.

Example using CSS:

<style>
  .line-break {
    display: block;
    margin-bottom: 1em;
  }
</style>

<p>
  Line one.<span class="line-break"></span>
  Line two.<span class="line-break"></span>
  Line three.
</p>