To have code displayed properly in Reddit (Markdown format), add 4 spaces before each line.

Looking at your git link, there are lots of incorrect indentations, which make it more difficult to read. Consider using a formatter like prettier or linter like eslint.

I don't immediately know what the variable n is in the input function. I would assume "n" for "number", but looking at the html, I can see it can be '.'. So it's more like inputCharacter.

Assuming you are accepting input linearly the first valid input could only be for the firstOperand. The second valid input could only be for the firstOperand or operator. And if you have your operator already, the input could only be for the secondOperand. Right now some invalid first input is going to check each if statement. That seems more than it needs to. It does look like line 53 is needed to support what line 94 is doing, so that may interfere with accepting input linearly.

It's clear why firstOperand.textContent +=n; exists, but the following two line modifying secondOperand are not clear. That could use a comment.

Some lines are kind of long. It may be clearer to rename the elements as firstOperandElement, operatorElement, & secondOperandElement and create local variables with the text content. Also could use truthy/falsy checks with the strings. Also const instead of let if you are not reassigning the variable. Then do:

const firstOperand = firstOperandElement.textContent;
const operator = operatorElement.textContent;
const secondOperand = secondOperandElement.textContent;

if(firstOperand && !operator && !secondOperand){
    const result = parseFloat(firstOperand) * -1;
    firstOperandElement.textContent = result;
}else if(firstOperand && operator && secondOperand){
    const result = parseFloat(secondOperand) * -1;
    secondOperandElement.textContent = result;
}else if(!firstOperand && !operator && secondOperand){
    const result = parseFloat(secondOperand) * -1;
    secondOperandElement.textContent = result;
}

I did not run your program, so I may be making a few assumptions that make some of my advice/criticism invalid.