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HTML 

              
                <p><a href="http://www.w3.org/TR/CSS2/syndata.html#value-def-identifier">The CSS Spec</a> says you can't start a class name with a number. HOWEVER you can start it with a backslash-escaped identifier. So ".2 {}" is invalid, but ".\32 {}" - the character sequence for '2' is valid. Thanks to some solid coincidences the codepoints for numbers begin at \30, so \30 is 0, \31 is 1, \32 is 2 etc. So if you want to start your class name with a number prefix it with \3 and you're golden assuming you want a single number as the initial value.</p>

<p>In case you need to start a class name with multiple numbers characters - such as if you want a numerator of "15" - then CSS needs to be able to tell the difference between "that character is an literal character" and "that character is part of an escape sequence". There are a few ways to do this. The following examples all represent the classname "15/24": you can escape every number (e.g. ".\31\35\/24"), you can add a space after the end of an escape sequence (e.g. ".\31 5\/24"), or you can have exactly six hex characters in the escape code (e.g. ".\0000315\/24"). Using all escape sequences feels cleanest to me when we're talking about just numbers.</p>

<p>In addition to that, you can also have all kinds of other characters <a href="https://mathiasbynens.be/notes/css-escapes">as long as you escape them</a>. In this instance we're using / to show numeric fractions and <a href="https://csswizardry.com/2015/08/bemit-taking-the-bem-naming-convention-a-step-further/#responsive-suffixes">@ to denote changes at a given breakpoint</a>.</p>

<p>The following is totally valid HTML and CSS:</p>

<hr>

<div class="grid">
  <div class="grid__item 1/1 1/3@large"><p><code>1/1 1/3@large</code><br>Full width as a baseline, one third at large sizes</p></div>
  <div class="grid__item 1/2 1/3@large"><p><code>1/2 1/3@large</code><br>One half width as a baseline, one third at large sizes</p></div>
  <div class="grid__item 1/2 1/3@large"><p><code>1/2 1/3@large</code><br>One half width as a baseline, one third at large sizes</p></div>
</div>

<div class="grid">
  <div class="grid__item 11/24"><p><code>11/24 </code><br>Slightly under half width as a baseline</p></div>
  <div class="grid__item 13/24"><p><code>11/24</code><br>Slightly over half width as a baseline, is bold and italic to demonstrate other ways of targeting numerators greater than or equal to 10</p></div>
</div>
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CSS 

              
                /* Base Widths */
.\31\/1 { width: 100%; }

.\31\/2 { width: 50%; }

.\31\/3 { width: 33.33333%; }
.\32\/3 { width: 66.66666%; }

/* Numbers bigger than 10 can be represented by escaping every non-hex character */
.\31\31\/24 { width: 45.83333% } /* 11/24 */
.\31\33\/24 { width: 54.16667% } /* 13/24 */

/* There are other ways to make it explicit when an escape sequence ends, both of these target "13/24" */
.\31 3\/24 { font-weight: bold; } /* a space deliniates the end of an escape sequence */
.\0000313\/24 { font-style: italic; } /* using 6 hex chars for the escape sequence */


/* Use the @ suffix to identify particular breakpoints */
@media (min-width: 600px) {
  .\31\/1\@large { width: 100%; }

  .\31\/2\@large { width: 50%; }

  .\31\/3\@large { width: 33.33333%; }
  .\32\/3\@large { width: 33.33333%; }
}



/* The simplest of grids systems */
.grid {
  margin-left: -8px;
  margin-right: 0;
  padding-left: 0;
  padding-right: 0;
  list-style: none;
  letter-spacing: -0.31em;
}

.opera:-o-prefocus,
.grid {
  word-spacing: -0.43em;
}


.grid__item {
  display: inline-block;
  box-sizing: border-box;
  padding-left: 8px;
  vertical-align: top;
  letter-spacing:normal;
  word-spacing:normal;
}

/* A splash of color */
.grid__item:nth-child(1) { color: red; }
.grid__item:nth-child(2) { color: green; }
.grid__item:nth-child(3) { color: blue; }
!

JS 

              
                console.log(document.querySelectorAll('.\\31\\/1'));
!
999px

Console