How to align two elements on the same line without changing HTML

I have two elements on the same line floated left and floated right.

<style type="text/css">
    #element1 {float:left;}
    #element2 {float:right;}
</style>

<div id="element1">
    element 1 markup
</div>
<div id="element2">
    element 2 markup
</div>

I need for element2 to line up next to element1 with about 10 pixels of padding between the two. The problem is that element2's width can change depending on content and browser (font size, etc.) so it's not always lined up perfectly with element1 (I can't just apply a margin-right and move it over).

I also cannot change the markup.

Is there a uniform way to line them up? I tried margin-right with a percentage, I tried a negative margin on element1 to bring element2 closer (but couldn't get it to work).

Using display:inline-block

#element1 {display:inline-block;margin-right:10px;} 
#element2 {display:inline-block;} 

Example

div {
  display: flex;
  justify-content: space-between;
}

<div>
  <p>Item one</p>
  <a>Item two</a>
</div>

#element1 {float:left;}
#element2 {padding-left : 20px; float:left;}

fiddle : http://jsfiddle.net/sKqZJ/

or

#element1 {float:left;}
#element2 {margin-left : 20px;float:left;}

fiddle : http://jsfiddle.net/sKqZJ/1/

or

#element1 {padding-right : 20px; float:left;}
#element2 {float:left;}

fiddle : http://jsfiddle.net/sKqZJ/2/

or

#element1 {margin-right : 20px; float:left;}
#element2 {float:left;}

fiddle : http://jsfiddle.net/sKqZJ/3/

reference : The Difference Between CSS Margins and Padding

By using display: inline-block; And more generally when you have a parent (always there is a parent except for html) use display: inline-block; for the inner elements. and to force them to stay in the same line even when the window get shrunk (contracted). Add for the parent the two property:

white-space: nowrap;
overflow-x: auto;

here a more formatted example to make it clear:

.parent {
    white-space: nowrap;
    overflow-x: auto;
}

.children {
   display: inline-block;
   margin-left: 20px; 
}

For this example particularly, you can apply the above as fellow (i'm supposing the parent is body. if not you put the right parent), you can also like change the html and add a parent for them if it's possible.

body { /*body may pose problem depend on you context, there is no better then have a specific parent*/
        white-space: nowrap;
        overflow-x: auto;
 }

#element1, #element2{ /*you can like put each one separately, if the margin for the first element is not wanted*/
   display: inline-block;
   margin-left: 10px; 
}

keep in mind that white-space: nowrap; and overlow-x: auto; is what you need to force them to be in one line. white-space: nowrap; disable wrapping. And overlow-x:auto; to activate scrolling, when the element get over the frame limit.

Change your css as below

#element1 {float:left;margin-right:10px;} 
#element2 {float:left;} 

Here is the JSFiddle http://jsfiddle.net/a4aME/

In cases where I use floated elements like that, I usually need to be sure that the container element will always be big enough for the widths of both floated elements plus the desired margin to all fit inside of it. The easiest way to do that is obviously to give both inner elements fixed widths that will fit correctly inside of the outer element like this:

#container {width: 960px;}
#element1  {float:left; width:745px; margin-right:15px;}
#element2  {float:right; width:200px;}

If you can't do that because this is a scaling width layout, another option is to have every set of dimensions be percentages like:

#element1 {float:left; width:70%; margin-right:10%}
#element2 {float:right; width:20%;}

This gets tricky where you need something like this:

#element1 {float:left; width:70%; margin-right:10%}
#element2 {float:right; width:200px;}

In cases like that, I find that sometimes the best option is to not use floats, and use relative/absolute positioning to get the same effect like this:

#container {position:relative;} /* So IE won't bork the absolute positioning of #element2 */
#element1 {margin-right:215px;}
#element2 {display: block; position:absolute; top:0; right:0; height:100%; width:200px;}

While this isn't a floated solution, it does result in side by side columns where they are the same height, and one can remain fluid with while the other has a static width.

This is what I used for similar type of use case as yours.

<style type="text/css">
#element1 {display:inline-block; width:45%; padding:10px}
#element2 {display:inline-block; width:45%; padding:10px}
</style>

<div id="element1">
 element 1 markup
</div>
<div id="element2">
 element 2 markup
</div>

Adjust your width and padding as per your requirement. Note - Do not exceed 'width' more than 100% altogether (ele1_width+ ele2_width) to add 'padding', keep it less than 100%.