I am not a double post, but there is a lot of information in the other - note that I posted as community wiki not to abuse punctuation
you know, posts marked as Community wiki, even if voted well, do not give points to the author.

Using borders

Just to complete the post, it follows a version with borders and pseudo-elements:

Basically it is a white border with dots, on a solid black border:

.selo {
  position:relative;
  display:inline-block;
  padding:20px;
  border:10px dotted #fff;
}

.selo:after {
  content:'';
  position:absolute; box-sizing: border-box;
  top:-5px; right:-5px; bottom: -5px; left:-5px; 
  border:10px solid #000;
  z-index: -1;
}

<div class="selo">
  Carta 1<br>
  00000-000 - São Paulo - SP
</div>

<div class="selo">
  Cartinha 2
</div>

option 1

Using outline and outline-offset the outline works as an embroidery and the offset puts that draw in a little, simulating the effect, the size of the border is controlled by outline-width and outline-color color. It is also responsive and does not lose resolution.

Browser support: link

Mozilla Documentation: link

div {
	width: 30%;
	height: 40%;
	position: relative;
	background-color: black;
	outline-style: dotted;
    outline-width: 2em;
    outline-color: #fff;
    outline-offset: -1em;
    float: left;
    margin: 30px;
}


div > section {
	position: relative;
	z-index: 2;
	box-sizing: border-box;
	padding: 40px;
	display: block;
	color: #fff;
}

<div>
	<section>
		Lorem ipsum dolor, sit amet consectetur adipisicing elit. Fuga labore illo rem ut. Iusto, temporibus laboriosam? Odio sapiente veritatis similique.
	</section>
</div>

<div style="outline-color: #f00; width: 300px; height: 200px">
	<section>
		Lorem ipsum dolor, sit amet consectetur adipisicing elit. Fuga labore illo rem ut. Iusto, temporibus laboriosam? Odio sapiente veritatis similique.
	</section>
</div>

OBS: Does not work in IE: (

Option 2

In addition to the option mentioned by @bacco another option would be using radial-gradient, so you do not need to use an image as a reference for the borders.

The positives , no image involved, no img request on the server, no resolution problem if you want to change the size and it is easy to change the color any time I want

Negative Points , is not as flexible as border-image in width x height , and is not as practical as maintenance of the code.

Follow the example. Notice that it's just a div that is the seal. it uses two pseudo elements. one with the pattern of the polka dots, and another to make the content box.

div {
	width: 300px;
	height: 400px;
	position: relative;
	/* border: 1px solid; */
	background-color: black;
}
div::before {
	content: "";
	position: absolute;
	top: 0;
	left: 0;
	width: 100%;
	height: 100%;
	/* background-color: aquamarine; */
	background-image: radial-gradient(circle, #fff 0%, #fff 49%, transparent 51%, transparent 100%);
    background-size: 50px 50px;
    background-position: -25px -25px;
}
div::after {
	content: "";
    position: absolute;
    top: 0;
    left: 0;
    width: calc(300px - 50px);
    height: calc(400px - 50px);
    background-color: #fff;
    right: 0;
    bottom: 0;
    margin: auto;
}

div > section {
	position: relative;
	z-index: 2;
	box-sizing: border-box;
	padding: 40px;
	display: block;
}

<div>
  <section>
    Lorem ipsum dolor, sit amet consectetur adipisicing elit. Fuga labore illo rem ut. Iusto, temporibus laboriosam? Odio sapiente veritatis similique.
  </section>
</div>

Browser support from IE10: link

Mozilla's radial-gradient documentation: link