How to insert a large block of HTML in JavaScript?

This answer does not use backticks/template literals/template strings (``), which are not supported by Internet Explorer.

Using HTML + JavaScript:

You could keep the HTML block in an invisible container (like a <script>) within your HTML code, then use its innerHTML at runtime in JS

For example:

// Create a temporary <div> to load into
var div = document.createElement('div');
div.setAttribute('class', 'someClass');
div.innerHTML = document.getElementById('blockOfStuff').innerHTML;

// You could optionally even do a little bit of string templating
div.innerHTML = div.innerHTML
    .replace(/{VENDOR}/g, 'ACME Inc.')
    .replace(/{PRODUCT}/g, 'Best TNT')
    .replace(/{PRICE}/g, '$1.49');

// Write the <div> to the HTML container
document.getElementById('targetElement').appendChild(div);

.red {
    color: red
}

<script id="blockOfStuff" type="text/html">
    Here's some random text.
    <h1>Including HTML markup</h1>
    And quotes too, or as one man said, "These are quotes, but
    'these' are quotes too."<br><br>
    <b>Vendor:</b> {VENDOR}<br>
    <b>Product:</b> {PRODUCT}<br>
    <b>Price:</b> {PRICE}
</script>

<div id="targetElement" class="red"></div>

Idea from this answer: JavaScript HERE-doc or other large-quoting mechanism?

Using PHP:

If you want to insert a particularly long block of HTML in PHP you can use the Nowdoc syntax, like so:

<?php

    $some_var = " - <b>isn't that awesome!</b>";

    echo
<<<EOT
    Here's some random text.
    <h1>Including HTML markup</h1>
    And quotes too, or as one man said, "These are quotes, but 'these' are quotes too."
    <br><br>
    The beauty of Nowdoc in PHP is that you can use variables too $some_var
    <br><br>
    Or even a value contained within an array - be it an array from a variable you've set
    yourself, or one of PHP's built-in arrays. E.g. a user's IP: {$_SERVER['REMOTE_ADDR']}
EOT;

?>

Here's a PHP Fiddle demo of the above code that you can run in your browser.

One important thing to note: The <<<EOT and EOT; MUST be on their own line, without any whitespace before them!

Why use Nowdoc in PHP?

One huge advantage of using Nowdoc syntax over the usual starting and stopping your PHP tag is its support for variables. Consider the normal way of doing it - shown in the example below:

<?php

    // Load of PHP code here

?>

Now here's some HTML...<br><br>

Let's pretend that this HTML block is actually a couple of hundred lines long, and we
need to insert loads of variables<br><br>

Hi <?php echo $first_name; ?>!<br><br>

I can see it's your birthday on <?php echo $birthday; ?>, what are you hoping to get?

<?php

    // Another big block of PHP here

?>

And some more HTML!
</body>
</html>

Contrast that to the simplicity of Nowdoc.

Template literals may solve your issue as it will allow writing multi-line strings and string interpolation features. You can use variables or expression inside string (as given below). It's easy to insert bulk html in a reader friendly way.

I have modified the example given in question and please see it below. I am not sure how much browser compatible Template literals are. Please read about Template literals here.

var a = 1, b = 2;
var div = document.createElement('div');
div.setAttribute('class', 'post block bc2');
div.innerHTML = `
    <div class="parent">
        <div class="child">${a}</div>
        <div class="child">+</div>
        <div class="child">${b}</div>
        <div class="child">=</div>
        <div class="child">${a + b}</div>
    </div>
`;
document.getElementById('posts').appendChild(div);

.parent {
  background-color: blue;
  display: flex;
  justify-content: center;
}
.post div {
  color: white;
  font-size: 2.5em;
  padding: 20px;
}

<div id="posts"></div>