carto-es6.js

// ES6 allows multiline strings with backticks, and it is natively available on all browsers except IE
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals
// just changing the docs to promote using ES6 by default (with an ES5 fallback) would already help a great deal
var countriesStyle = new carto.style.CartoCSS(`
  #layer {
    polygon-fill: #333333;
  }
`);

carto-object.js

// baked in simple object literal to multiline string conversion shouldn't be to hard to implement
var countriesStyle = new carto.style.CartoCSS({
  '#layer': {
    'polygon-fill': '#333333'
  }
});

// and this would also allows for interesting scenarios with dynamically generated CartoCSS
// (again using an ES6 feature, see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Object_initializer)
var layerName = '#layer';
var num = 42;
var countriesStyle = new carto.style.CartoCSS({
  [layerName]: {
    'polygon-fill': '#333333'
  }
  [`someLayer${num}`]: {
    'polygon-fill': '#333333'
  }
});