Inline elements shifting when made bold on hover

Question

Pre-set the width by using an invisible pseudo-element which has the same content and styling as the parent hover style. Use a data attribute, like title, as the source for content.

li {
    display: inline-block;
    font-size: 0;
}

li a {
    display:inline-block;
    text-align:center;
    font: normal 16px Arial;
    text-transform: uppercase;
}

a:hover {
    font-weight:bold;
}

/* SOLUTION */
/* The pseudo element has the same content and hover style, so it pre-sets the width of the element and visibility: hidden hides the pseudo element from actual view. */
a::before {
    display: block;
    content: attr(title);
    font-weight: bold;
    height: 0;
    overflow: hidden;
    visibility: hidden;
}

<ul>
    <li><a href="#" title="height">height</a></li>
    <li><a href="#" title="icon">icon</a></li>
    <li><a href="#" title="left">left</a></li>
    <li><a href="#" title="letter-spacing">letter-spacing</a></li>
    <li><a href="#" title="line-height">line-height</a></li>
</ul>

A compromised solution is to fake bold with text-shadow, e.g:

text-shadow: 0 0 0.01px black;

For better comparison I created these examples:

a, li {
  color: black;
  text-decoration: none;
  font: 18px sans-serif;
  letter-spacing: 0.03em;
}
li {
  display: inline-block;
  margin-right: 20px;
  color: gray;
  font-size: 0.7em;
}
.bold-x1 a.hover:hover,
.bold-x1 a:not(.hover) {
  text-shadow: 0 0 .01px black;
}
.bold-x2 a.hover:hover,
.bold-x2 a:not(.hover){
  text-shadow: 0 0 .01px black, 0 0 .01px black;
}
.bold-x3 a.hover:hover,
.bold-x3 a:not(.hover){
  text-shadow: 0 0 .01px black, 0 0 .01px black, 0 0 .01px black;
}
.bold-native a.hover:hover,
.bold-native a:not(.hover){
  font-weight: bold;
}

.bold-native li:nth-child(4),
.bold-native li:nth-child(5){
 margin-left: -6px;
 letter-spacing: 0.01em;
}

<ul class="bold-x1">
  <li><a class="hover" href="#">Home</a></li>
  <li><a class="hover" href="#">Products</a></li>
  <li><a href="#">Contact</a></li>
  <li><a href="#">About</a></li>
  <li>Bold (text-shadow x1)</li>
</ul>
<ul class="bold-x2">
  <li><a class="hover" href="#">Home</a></li>
  <li><a class="hover" href="#">Products</a></li>
  <li><a href="#">Contact</a></li>
  <li><a href="#">About</a></li>
  <li>Extra Bold (text-shadow x2)</li>
</ul>
<ul class="bold-native">
  <li><a class="hover" href="#">Home</a></li>
  <li><a class="hover" href="#">Products</a></li>
  <li><a href="#">Contact</a></li>
  <li><a href="#">About</a></li>
  <li>Bold (native)</li>
</ul>
<ul class="bold-x3">
  <li><a class="hover" href="#">Home</a></li>
  <li><a class="hover" href="#">Products</a></li>
  <li><a href="#">Contact</a></li>
  <li><a href="#">About</a></li>
  <li>Black (text-shadow x3)</li>
</ul>

Passing to text-shadow really low value for blur-radius will make the blurring effect not so apparent.

In general the more your repeat text-shadow the bolder your text will get but in the same time loosing original shape of the letters.

I should warn you that setting the blur-radius to fractions is not going to render the same in all browsers! Safari for example need bigger values to render it the same way Chrome will do.

Another idea is using letter-spacing

li, a { display: inline-block; }
a {
  font-size: 14px;
  padding-left: 10px;
  padding-right: 10px;
  letter-spacing: 0.235px
}

a:hover, a:focus {
  font-weight: bold;
  letter-spacing: 0
}

<ul>
  <li><a href="#">item 1</a></li>
  <li><a href="#">item 2</a></li>
  <li><a href="#">item 3</a></li>
</ul>

If you cannot set the width, then that means the width will change as the text gets bold. There is no way to avoid this, except by compromises such as modifying the padding/margins for each state.

One line in jquery:

$('ul.nav li a').each(function(){
    $(this).parent().width($(this).width() + 4);
});

edit: While this can bring about the solution, one should mention that it does not work in conjunction with the code in the original post. "display:inline" has to be replaced with floating-parameters for a width-setting to be effective and that horizontal menu to work as intended.

CSS3 Solution - Experimental

(Fake boldness)

Thought to share a different solution which no one suggested here. There's a property called text-stroke which will be handy for this.

p span:hover {
  -webkit-text-stroke: 1px black;
}

<p>Some stuff, <span>hover this,</span> it's cool</p>

Here, I am targeting the text inside of the span tag and we stroke it by a pixel with black which will simulate bold style for that particular text.

Note that this property is not widely supported, as of now (while writing this answer), only Chrome and Firefox seems to support this. For more information on browser support for text-stroke, you can refer to CanIUse.

---

Just for sharing some extra stuff, you can use -webkit-text-stroke-width: 1px; if you are not looking to set a color for your stroke.