JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container"></div>

CSS

.processed {
    background-color: blue;
}

JavaScript

// preprocessing code with jQuery

var codeToProcess = "<div>" + 
                    "    <div id='myDiv1'>a</div>" + 
                    "    <div id='myDiv2'>b</div>" + 
                    "    <div id='myDiv3'>c</div>" + 
                    "</div>";

var $toProcess = $( codeToProcess );
$toProcess.find( "div" ).addClass( "processed" );

// getting by id before insertion
alert( $toProcess.find( "#myDiv1" ).html() );
alert( $( "#myDiv1" ).html() ); // this will return null, since the divs 
                                // were not yet added to the document

$toProcess.appendTo( "#container" );
// or $( "#container" ).html( $toProcess );

// getting by id after insertion
alert( $( "#myDiv2" ).html() );