Perhaps a CSS Monoid

css.js

function Unstyled() {
  this.append = function (other) {
    return other;
  };

  this.empty = function () {
    return new Unstyled();
  };

  this.$addLesserStyle = function (style, value) {
    return new SingleStyle(style, value);
  };

  this.toString = function () {
    return "{\n}";
  };
}

function SingleStyle(style, value) {
  this.append = function (other) {
    return other.$addLesserStyle(style, value);
  };

  this.empty = function () {
    return new Unstyled();
  };

  this.$addLesserStyle = function (newStyle, newValue) {
    if (newStyle === style) {
      return new SingleStyle(style, value);
    } else {
      return new MultiStyle({
        [style]: value,
        [newStyle]: newValue
      });
    }
  };

  this.toString = function () {
    return "{\n  " + style + ": " + value + ";\n}";
  };
}

function MultiStyle(styleDefs) {
  this.append = function (other) {
    var result = other;
    for (var style in styleDefs) {
      result = result.$addLesserStyle(style, styleDefs[style]);
    }
    return result;
  };

  this.empty = function () {
    return new Unstyled();
  };

  this.$addLesserStyle = function (newStyle, newValue) {
    var result = { [newStyle]: newValue };
    for (var style in styleDefs) {
      result[style] = styleDefs[style];
    }
    return new MultiStyle(result);
  };

  this.toString = function () {
    var result = '';
    for (var style in styleDefs) {
      result +=  '  ' + style + ': ' + styleDefs[style] + ';\n';
    }
    return "{\n" + result + "}";
  };
}

function unstyled() {
  return new Unstyled();
}

function style(name, value) {
  return new SingleStyle(name, value);
}

function styles(obj) {
  var keys = Object.keys(obj);

  switch (keys.length) {
    case 0:
      return unstyled();
    case 1:
      return style(keys[0], obj[keys[0]]);
    default:
      return new MultiStyle(obj);
  }
}

function concat(styles) {
  return styles.reduce(
    function (acc, current) { return acc.append(current); },
    unstyled()
  );
}

function color(value) {
  return new SingleStyle('color', value);
}

function backgroundColor(value) {
  return new SingleStyle('background-color', value);
}

console.log(new Unstyled().toString());
console.log(new SingleStyle('color', 'red').toString());
console.log(color('red').append(color('yellow')).append(backgroundColor('black')).toString());

var blackOnYellow = color('black').append(backgroundColor('yellow'));

console.log(
  blackOnYellow.toString()
);

console.log(
  styles({})
);

console.log(
  styles({ color: 'red' })
);

console.log(
  styles({ color: 'red', 'background-color': 'green' })
    .append(
      styles({ color: 'blue', 'background-color': 'pink' })).toString()
);