Indent first line of a paragraph, but only if it takes multiple lines

Question

I'd like to indent the first line of paragraphs (exactly what the text-indent CSS property does), but only for those paragraphs that take multiple lines. If a paragraph fits in a single line, I don't want it to be indented.

Is it possible to achieve a similar result?

p {
 text-indent: 20px;
 max-width: 350px;
}

<p>This is a pretty long paragraph which spans over a few lines. For that reason I wish its first line to be indented (as it should be).</p>

<p>These are short paragraphs.</p>

<p>I don't want them indented.</p>

<p>Unless they take multiple lines.</p>

Answer 1

Here is hacky idea that requires an extra wrapper:

p {
 max-width: 350px;
 line-height:1.2em; /* height of one line */
}
.e {
  display:flex; /* a flex container to be able to use 100% in the height*/
}
/* a pseudo element to create the indentation*/
.e p:before {
  content:"";
  float:left;
  width:20px; 
  /* 0px if 100% (height of p) is less or equal to 1.2em (one line)
     1px if 100% is bigger than one line
  */
  height:clamp(0px,100% - 1.2em,1px)
}

<div class="e"><p>This is a pretty long paragraph which spans over a few lines. For that reason I wish its first line to be indented (as it should be).</p></div>

<div class="e"><p>These are short paragraphs.</p></div>

<div class="e"><p>I don't want them indented.</p></div>

<div class="e"><p>Unless they take multiple lines.</p></div>

An interactive demo where you can resize to see the magic in play:

p {
 line-height:1.2em; /* height of one line */
}
.e {
  display:flex; /* a flex container to be able to use 100% in the height*/
  overflow:hidden;
  border:1px solid;
  resize:horizontal;
  width: 350px;
}
/* a pseudo element to create the indentation*/
.e p:before {
  content:"";
  float:left;
  width:20px; 
  /* 0px if 100% (height of p) is less or equal to 1.2em (one line)
     1px if 100% is bigger than one line
  */
  height:clamp(0px,100% - 1.2em,1px)
}

<div class="e"><p>This is a pretty long paragraph which spans over a few lines. For that reason I wish its first line to be indented (as it should be).</p></div>