Why Is There an Unexplainable Gap Between These Inline-Block Div Elements

.parent {    display: flex;}.parent > div {    display: inline-block;    padding: 1em;    border: 2px solid #f00;}

<div class="parent">    <div>text</div>    <div>text</div>    <div>text</div>    <div>text</div>    <div>text</div></div>

Extra Space Between inline-block DIV Elements

You need to comment out the white space between the elements, like so:

<div class="tile-container">
       <div class="tile-large">1</div><!--
    --><div class="tile-wide">2</div><!--
    --><div class="tile-small">3</div><!--
    --><div class="tile-small">4</div><!--
    --><div class="tile-small">5</div><!--
    --><div class="tile-small">6</div><!--
    --><div class="tile-wide">7</div><!--
    --><div class="tile-large">8</div>
</div>

DEMO: http://jsfiddle.net/P7cbA/11/

Another way is to put the elements in the HTML in one line without any spaces between them, but this reduces the code readability:

<div class="tile-container">
       <div class="tile-large">1</div><div class="tile-wide">2</div><div class="tile-small">3</div><div class="tile-small">4</div><div class="tile-small">5</div><div class="tile-small">6</div><div class="tile-wide">7</div><div class="tile-large">8</div>
</div>

How to remove the space between inline/inline-block elements?

Alternatively, you should now use flexbox to achieve many of the layouts that you may previously have used inline-block for: https://css-tricks.com/snippets/css/a-guide-to-flexbox/

Since this answer has become rather popular, I'm rewriting it significantly.

Let's not forget the actual question that was asked:

How to remove the space between inline-block elements? I was hoping
for a CSS solution that doesn't require the HTML source code to be
tampered with. Can this issue be solved with CSS alone?

It is possible to solve this problem with CSS alone, but there are no completely robust CSS fixes.

The solution I had in my initial answer was to add font-size: 0 to the parent element, and then declare a sensible font-size on the children.

http://jsfiddle.net/thirtydot/dGHFV/1361/

This works in recent versions of all modern browsers. It works in IE8. It does not work in Safari 5, but it does work in Safari 6. Safari 5 is nearly a dead browser (0.33%, August 2015).

Most of the possible issues with relative font sizes are not complicated to fix.

However, while this is a reasonable solution if you specifically need a CSS only fix, it's not what I recommend if you're free to change your HTML (as most of us are).

This is what I, as a reasonably experienced web developer, actually do to solve this problem:

<p>
    <span>Foo</span><span>Bar</span>
</p>

Yes, that's right. I remove the whitespace in the HTML between the inline-block elements.

It's easy. It's simple. It works everywhere. It's the pragmatic solution.

You do sometimes have to carefully consider where whitespace will come from. Will appending another element with JavaScript add whitespace? No, not if you do it properly.

Let's go on a magical journey of different ways to remove the whitespace, with some new HTML:

<ul>
    <li>Item 1</li>
    <li>Item 2</li>
    <li>Item 3</li>
</ul>

• You can do this, as I usually do:

   <ul>
       <li>Item 1</li><li>Item 2</li><li>Item 3</li>
   </ul>
  

http://jsfiddle.net/thirtydot/dGHFV/1362/

• Or, this:

   <ul>
       <li>Item 1</li
       ><li>Item 2</li
       ><li>Item 3</li>
   </ul>
  

• Or, use comments:

   <ul>
       <li>Item 1</li><!--
       --><li>Item 2</li><!--
       --><li>Item 3</li>
   </ul>
  

• Or, if you are using using PHP or similar:

   <ul>
       <li>Item 1</li><?
       ?><li>Item 2</li><?
       ?><li>Item 3</li>
   </ul>
  

• Or, you can even skip certain closing tags entirely (all browsers are fine with this):

   <ul>
       <li>Item 1
       <li>Item 2
       <li>Item 3
   </ul>
  

Now that I've gone and bored you to death with "one thousand different ways to remove whitespace, by thirtydot", hopefully you've forgotten all about font-size: 0.

Unexplained Space Between Divs

It's the automatic margin-before on the <p> tag that is applied by most browsers. Set:

#black p {
    margin: 0
}

and you'll see it go away.

Why does the extra space between two inline-elements get filled with background-color, but it doesn't for the space between two inline-block elements?

Your answer is within the specification.

For each inline (including anonymous inlines; see [CSS2] section 9.2.2.1) within an inline formatting context, ...

Any collapsible space immediately following another collapsible space—even one outside the boundary of the inline containing that space, provided both spaces are within the same inline formatting context—is collapsed to have zero advance width. (It is invisible, but retains its soft wrap opportunity, if any.)

It's a bit complex but in the case of inline element the spaces will collapse into one space that will be inside the first inline element and not between both inline element. This is because you had a space at the end of your inline element. In other words, all the spaces will collapse into the first one and the position of the first space will decide about the visual.

Remove the space at the end of your inline elements and you will have a different result:

.div1 {
  display: inline;
  background-color: lightblue;
}

.div2 {
  display: inline;
  background-color: orange;
}

<div class="box">
  <div class="div1"> Lorem ipsum </div>
  <div class="div2"> Lorem ipsum </div>
</div>
<div class="box">
  <div class="div1"> Lorem ipsum</div>
  <div class="div2"> Lorem ipsum </div>
</div>

<div class="child child-1">ONE</div><div class="child child-2">TWO</div>

or

<div class="child child-1">ONE</div
><div class="child child-2">TWO</div>

span {
  display: inline-block;
}

<span>This will have</span>
<span>a gap between the elements</span>

<span>This won't have</span><span>a gap between the elements</span>

<div>Test</div>
<div>Test</div>

<div>Test</div><div>Test</div>

<div>Test</div><!--
--><div>Test</div>

<div id="left">
    left other elements goes here
</div>
<div id="center">
    center other elements goes here
</div>

<div id="left">
    left other elements goes here
</div><div id="center">
    center other elements goes here
</div>