<div class="classifier">
		<div class="classifier-left" id="1" style="widgt: 700px;">...</div>
                <div class="classifier-right" id="2" style="widgt: 850px;">...</div>
</div> 

.classifier-left {
	overflow: auto; 
	resize: horizontal;
	float: left;
	display : table-row;
}
.classifier-right {
	display : table-row;
}

frame.onresize = function() {...}

 Object.observe(document.getElementById('1'), function (changes) {
          console.log("changes", changes);
      });

Uncaught TypeError: Object.observe is not a function

function outputsize() {
 width.value = classifier.offsetWidth
 height.value = classifier.offsetHeight
}
outputsize()

new ResizeObserver(outputsize).observe(мой_объект)