Q:

Using br or div for section break

I've worked at few places and seen two different methods of doing a section or line break in HTML.  
One way I've seen it done is like this:
<div class="placeholder-100pct">&nbsp;</div>

And the other is just using plain old <br />.
Any benefit to one over the other or would it be just a matter of style?

A:

Use <br/> when you want a new line in a paragraph, like so:
<p>Hi Josh, <br/> How are you?</p>

This might be useful when writing an address:
<p>John Dough<br/>
1155 Saint. St. #33<br/>
Orlando, FL 32765
</p>

Using a div will automatically give you a new line, but if you want a space between two elements, you can use a margin on the div. 
Do not use <br/> to get some space between two divs:
<!-- This is not preferred -->
<div>Hello</div>
<br/>
<div>Something else here</div>

Hope this helps

A:

A div is a generic container. A br is a line break. Neither is particularly well suited for expressing a section break.
HTML 5 introduces sections. You mark up each section, not the break between them.
<section>
  <!-- This is a section -->
</section>
<section>
  <!-- This is another section -->
</section>

A:

Use a <br /> when it makes semantic sense to do so. If all you need is a line-break, that's what it's there for. If you're trying to split sections of different types of content, then each section should be in a container of its own. For example, if you have an address where each line of the address would show on a separate line, it would make sense to do:
<address>
123 Main Street<br />
Anywhere, USA 12345
</address>

