plugin-less instantiates CSSPluginBase() when it is not running in a browser.

[mikol-styra]

I am trying (unsuccessfully) to use plugin-less in conjunction with server-side rendering. The root cause of the problem is CSSPluginBase.prototype.instantiate(), which attempts to inject a style element into the DOM without document being defined:

CSSPluginBase.prototype.instantiate = function(load) {
  if (this.builder)
    return;

  var style = document.createElement('style');
  style.type = 'text/css';
  style.innerHTML = load.metadata.style;
  document.head.appendChild(style);
};

module.exports = CSSPluginBase;

I am not sure what the correct fix is in this case. There may be two. First, explicitly checking if document is defined in CSSPluginBase.prototype.instantiate() seems appropriate and works for me in practice:

  if (this.builder || typeof document === 'undefined')
    return;

Second, less.js may require a fix so that the CSSPluginBase() constructor is not even called unless plugin-less is running in a browser. plugin-css explicitly checks if it is running in a browser, for example, and does not have an issue with server-side rendering. Neither does mobilexag/plugin-sass.

[guybedford]

It should be fine to entirely short-circuit the CSS loading when running in a non-browser environment, with a check exactly like that.

If you think that will do it, please do send a PR. Or do we need to do something here as well?

[mikol-styra]

Will be fixed by systemjs/plugin-css/pull/113. After thinking about this for a bit, I like the fact that plugins built on top of CSSPluginBase are not required to worry about the browser injection issue.