Place a button right aligned

Question

Which alignment technique you use depends on your circumstances but the basic one is float: right;:

<input type="button" value="Click Me" style="float: right;">

You'll probably want to clear your floats though but that can be done with overflow:hidden on the parent container or an explicit <div style="clear: both;"></div> at the bottom of the container.

For example: http://jsfiddle.net/ambiguous/8UvVg/

Floated elements are removed from the normal document flow so they can overflow their parent's boundary and mess up the parent's height, the clear:both CSS takes care of that (as does overflow:hidden). Play around with the JSFiddle example I added to see how floating and clearing behave (you'll want to drop the overflow:hidden first though).

If the button is the only element on the block:

.border {
  border: 2px blue dashed;
}

.mr-0 {
  margin-right: 0;
}
.ml-auto {
  margin-left:auto;
}
.d-block {
  display:block;
}

<p class="border">
  <input type="button" class="d-block mr-0 ml-auto" value="The Button">
</p>

If there are other elements on the block:

.border {
  border: 2px indigo dashed;
}

.d-table {
  display:table;
}

.d-table-cell {
  display:table-cell;
}

.w-100 {
  width: 100%;
}

.tar {
  text-align: right;
}

<div class="border d-table w-100">
  <p class="d-table-cell">The paragraph.....lorem ipsum...etc.</p>
  <div class="d-table-cell tar">
    <button >The Button</button>
  </div>
</div>

With flex-box:

.flex-box {
  display:flex;
  justify-content:space-between;
  outline: 2px dashed blue;
}

.flex-box-2 {
  display:flex;
  justify-content: flex-end;
  outline: 2px deeppink dashed;
}

<h1>Button with Text</h1>
<div class="flex-box">
<p>Once upon a time in a ...</p>
<button>Read More...</button>
</div>

<h1>Only Button</h1>
<div class="flex-box-2">
  <button>The Button</button>
</div>

<h1>Multiple Buttons</h1>
<div class="flex-box-2">
  <button>Button 1</button>
  <button>Button 2</button>
</div>

Good Luck...

Another possibility is to use an absolute positioning oriented to the right:

<input type="button" value="Click Me" style="position: absolute; right: 0;">

Here's an example: https://jsfiddle.net/a2Ld1xse/

This solution has its downsides, but there are use cases where it's very useful.

a modern approach in 2019 using flex-box

with div tag

<div style="display:flex; justify-content:flex-end; width:100%; padding:0;">
    <input type="button" value="Click Me"/>
</div>

with span tag

<span style="display:flex; justify-content:flex-end; width:100%; padding:0;">
    <input type="button" value="Click Me"/>
</span>