Warning: Unknown DOM property class. Did you mean className?

Question

Yes, its a React convention:

Since JSX is JavaScript, identifiers such as class and for are discouraged as XML attribute names. Instead, React DOM components expect DOM property names like className and htmlFor, respectively.

JSX In Depth.

In React.js there is no for and class properties available so we need to use

for   --> htmlFor
class --> className

Meteor uses babel to transpile ES5 to ES6 (ES2015), so we can deal with it as a normal Node application with babel transpiler added.

You need to add .babelrc file to your project's root folder, and add the following items

{
  "plugins": [
    "react-html-attrs"
  ]
}

And of course, you need to install this plugin using npm:

npm install --save-dev babel-plugin-react-html-attrs

In HTML we use

class="navbar"

but in React and ReactNative there is no HTML, we use JSX(Javascript XML) here we use

className="navbar"

You can't use class for any HTML tag attribute because JS has another meaning of class. That why you have to use className instead of class.

And also use htmlFor attribute instead of for.

In react 16.13.1 you can use class attribute. It throws a "warning" exception but it don'T stop the execution.