'grid' code works right in brackets but not in DW

I'm learning both HTML and Dreamweaver. I copied a small amount of code directly out of a book and it works as intended in Brackets, but not in Dreamweaver. It is a few lines for creating a grid. I intended a grid layout of five lines of four boxes across, each. That's how it looks in Brackets viewing of page. But in DW, it comes out as twenty lines, vertically stacked. Here's the code:

HTML:

<main>

          <div>Div 1</div><div>Div 2</div><div>Div 3</div><div>Div 4</div>

        <div>Div 5</div><div>Div 6</div><div>Div 7</div><div>Div 8</div>

        <div>Div 9</div><div>Div 10</div><div>Div 11</div><div>Div 12</div>

        <div>Div 13</div><div>Div 14</div><div>Div 15</div><div>Div 16</div>

        <div>Div 17</div><div>Div 18</div><div>Div 19</div><div>Div 20</div>

</main>

CSS: