Responsive Separator for Horizontal List

This question expands upon 'Separators For Navigation' by asking, how it is possible to remove the separators at the line breaks cause by viewport size.

Wide Viewport

->       Item 1 | Item 2 | Item 3 | Item 4 | Item 5       <-

Small Viewport

->  Item 1 | Item 2 | Item 3  <-
->      Item 4 | Item 5       <-

Here is a fiddle that shows how a pipe remains at the line break:

Fiddle.

I'm interested in a css-only solution, but javascript is acceptable if it provides the only possible solution.

Solution

Explanation

You can exploit fact that trailing and line trailing white space automatically collapses:

document.write(
 'word<b style="background: red; outline: 1px solid blue;"> </b>'
 .repeat(42)
);

As you can see there are red spaces with blue outlines between words, but the very last and and two at line ends lack the red area because it's width collapsed to zero: that is the white-space collapsing in effect.

It is possible to adjust width with word-spacing and use pseudo element instead, so setting inline ::after { content: ' '; word-spacing: 2em; } gives you wide inline rectangle that can have decorated backgrounds or borders but disappears when it is not between words.

Simplified example

Simplified use case (from https://codepen.io/myf/pen/dyOzpZM, tested just in 2021-02 evergreen Firefox and Chromium, will not work in pre-Chromium Edge; for more robust example see the second snippet below):

ul {
  text-align: center;
  padding: 0;
}
li {
  display: inline;
}
li::after {
  /*
   This has to be space, tab or other
   breakable white-space character:
  */
  content: " ";
  word-spacing: 1em;
  background-image: linear-gradient(
    -0.2turn,
    transparent 0 calc(50% - 0.03em),
    currentcolor 0 calc(50% + 0.03em),
    transparent 0
  );
}
/*
 That's it: just inline text
 with styled ::after spaces
 that collapse at line breaks
 and at the end of the element.
 
 That's basically how spaces work in text.
*/

/*
 Unrelated whimsical effects:
*/
body { background: #456; color: #fed; min-height: 100vh; margin: 0; display: flex; align-items: center; }
ul { --dur: 3s; font-family: Georgia, serif; font-size: min(7vw, calc(100vh / 7)); margin: 0 auto; position: relative; padding: 0 1em; -webkit-text-fill-color: #999; text-transform: capitalize; animation: poing var(--dur) infinite alternate ease-in-out; }
@keyframes poing { from { max-width: 3.4em; } to { max-width: min(19em, calc(100vw - 2em)); color: lime; } }
ul::before, ul::after { -webkit-text-fill-color: currentcolor; position: absolute; top: 50%; transform: translatey(-50%); animation: calc(var(--dur) * 2) calc(var(--dur) * -1.5) infinite forwards linear; }
ul::before { content: "☜"; left: 0; animation-name: a !important; }
ul::after { content: "☞"; right: 0; animation-name: b !important; }
@keyframes a { 50% { content: "☛"; } }
@keyframes b { 50% { content: "☚"; } }
ul:hover, ul:hover::before, ul:hover::after { animation-play-state: paused; }

<ul>
 <li>foo</li>
 <li>bar</li>
 <li>baz</li>
 <li>gazonk</li>
 <li>qux</li>
 <li>quux</li>
</ul>

It uses flat list with single word items, so is not very relevant for real-world usage.

More realistic example with elements highlights

nav {
  text-align: center;
  padding-right: 1em; /* = li::after@word-spacing */
}
ul {
  display: inline;
  margin: 0;
  padding: 0;
}
li {
  display: inline;
  /*
   white-space: nowrap should be moved to child A
   because IE fails to wrap resulting list completely
  */
}
li::before {
  content: ' ';
  /*
   this content is important only for Chrome in case
   the HTML will be minified with *no whitespaces* between </li><li>
  */
}
li::after {
  content: ' ';
  /*
   this is actual placeholder for background-image
   and it really must be space (or tab)
  */
  white-space: normal;
  word-spacing: 1em;
  /*
   = nav@padding-right - this actually makes width
  */
  background-image: radial-gradient(circle, black, black 7%, transparent 15%, transparent 35%, black 45%, black 48%, transparent 55%);
  background-size: 1em 1em;
  background-repeat: no-repeat;
  background-position: center center;
  opacity: 0.5;
}
/*
 no need to unset content of li:last-child::after
 because last (trailing) space collapses anyway
*/
a {
  white-space: nowrap;
  display: inline-block; /* for padding */
  padding: 1em;
  text-decoration: none;
  color: black;
  transition-property: background-color;
  transition-duration: 500ms;
}
a:hover {
  background-color: #ccc;
}
/*
 For demonstrative purposes only
 Give items some content and uneven width
*/
nav:hover > ul > li {
  outline: 3px dotted rgba(0,0,255,.5);
  outline-offset: -3px;
}
nav:hover > ul > li::after {
  opacity: 1;
  background-color: rgba(255, 0, 0, .5);
}
nav:hover > ul > li:hover {
  outline-style: solid;
}
nav:hover > ul > li:hover::after  {
  background-color: cyan;
}

nav:hover > ul > li > a {
  outline: 3px solid rgba(0,255,0,.5);
  outline-offset: -3px;
}

nav > ul {
  counter-reset: c;
}
nav > ul > li {
  counter-increment: c;
}
nav > ul > li > a::before {
  content: counter(c, upper-roman) '. ';
  letter-spacing: .3em;
}
nav > ul > li > a::after {
  content: ' item ' counter(c, lower-roman);
  word-spacing: .3em;
  letter-spacing: .1em;
  transform: translatex(.1em);
  display: inline-block;
}

<nav>
  <ul><li><a href="#"></a></li><li><a href="#"></a></li><li><a href="#"></a></li><li><a href="#"></a></li><li><a href="#"></a></li><li><a href="#"></a></li><li><a href="#"></a></li><li><a href="#"></a></li><li><a href="#"></a></li><li><a href="#"></a></li><li><a href="#"></a></li><li><a href="#"></a></li><li><a href="#"></a></li><li><a href="#"></a></li><li><a href="#"></a></li><li><a href="#"></a></li><li><a href="#"></a></li><li><a href="#"></a></li><li><a href="#"></a></li><li><a href="#"></a></li><li><a href="#"></a></li>
  </ul>
</nav>
<!--  For demonstrative purposes is content of links made by CSS
-->

(Originally from https://jsfiddle.net/vnudrsh6/7/) This proof-of-concept uses background-image of "eventually colapsing" CSS generated content space after each <li>. Tested in 2016 in Firefox, Chrome and IE11.

Obviously you might need to use some character or more complex shape as divider. Naturally you can use (vector) background-image, and you can even use text in SVG, although making it correspond with surrounding ("real") text might be quite daunting.

Bare-bones with SVG

Minimal working example without any "list" element, with textual ❦ fleuron:

body {
  text-align: center;
}
b::after {
  content: " ";
  word-spacing: 16px;
  background: url("data:image/svg+xml;charset=utf-8,\
  <svg xmlns='http://www.w3.org/2000/svg' \
    viewBox='-3,-15,16,16'>\
      <text>❦</text>\
  </svg>");
}

<b>foo</b> <b>bar</b> <b>baz</b> <b>gazonk</b> <b>qux</b> <b>quux</b> 
<b>foo</b> <b>bar</b> <b>baz</b> <b>gazonk</b> <b>qux</b> <b>quux</b> 
<b>foo</b> <b>bar</b> <b>baz</b> <b>gazonk</b> <b>qux</b> <b>quux</b>

If you need to have spaced items to the sides (justify), text-align-last in addition to text-align may come in handy. Conveniently, it's the white space "character" yet again what is stretched.

ul {
  text-align: center;
  padding: 0;
  font-size: 30px;
  text-align: justify;
  text-align-last: justify;
  overflow: hidden;
  resize: both
}

li {
  display: inline;
}

li::after {
  content: " ";
  background-color: highlight;
  word-spacing: 3em;
  background-image: linear-gradient( 0.25turn, transparent 0 calc(50% - 0.03em), mark 0 calc(50% + 0.03em), transparent 0);
}

html {
  color-scheme: light dark;
}

<ul>
  <li>foo</li>
  <li>bar</li>
  <li>baz</li>
  <li>gazonk</li>
  <li>qux</li>
  <li>quux</li>
</ul>

Responsive Separator for Horizontal List

Explanation

Simplified example

More realistic example with elements highlights

Bare-bones with SVG

Other notable answers: