How can I force a long string without any blank to be wrapped?

for block elements:

<textarea style="width:100px; word-wrap:break-word;">
  ACTGATCGAGCTGAAGCGCAGTGCGATGCTTCGATGATGCTGACGATGCTACGATGCGAGCATCTACGATCAGTC
</textarea>

for inline elements:

<span style="width:100px; word-wrap:break-word; display:inline-block;"> 
   ACTGATCGAGCTGAAGCGCAGTGCGATGCTTCGATGATGCTGACGATGCTACGATGCGAGCATCTACGATCAGTC
</span>

Place zero-width spaces at the points where you want to allow breaks. The zero-width space is ​ in HTML. For example:

ACTGATCG​AGCTGAAG​CGCAGTGC​GATGCTTC​GATGATGC

I don't think you can do this with CSS. Instead, at regular 'word lengths' along the string, insert an HTML soft-hyphen:

ACTGATCG­AGCTGAAG­CGCAGTGC­GATGCTTC­GATGATGC­TGACGATG

This will display a hyphen at the end of the line, where it wraps, which may or may not be what you want.

Note Safari seems to wrap the long string in a <textarea> anyway, unlike Firefox.

If you're using PHP then the wordwrap function works well for this: http://php.net/manual/en/function.wordwrap.php

The CSS solution word-wrap: break-word; does not seem to be consistent across all browsers.

Other server-side languages have similar functions - or can be hand built.

Here's how the the PHP wordwrap function works:

$string = "ACTGATCGAGCTGAAGCGCAGTGCGATGCTTCGATGATGCTGACGATGCTACGATGCGAGCATCTACGATCAGTCGATGTAGCTAGTAGCATGTAGTGA";

$wrappedstring = wordwrap($string,50,"<br>",true);

This wraps the string at 50 characters with a <br> tag. The 'true' parameter forces the string to be cut.

In case if you use Bootstrap, better case for you is use this class "text-break".

Example:

<p class="text-break">mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm</p>

More informationg you should get in official Bootstrap documentation page

Simply:

word-break: break-word;

Here is the code I come into using white-space: pre-wrap;

.code {
    width: 90vw;
    white-space: pre-wrap;
    font-family: inherit;
    word-break: break-word;
    overflow-wrap: break-word;
    line-break: auto;
}

I know the with value is looks odd,you should change it to value fits your needs .