ESP8266 12E On board LEDs

const int led_pins[] = {D0, D4};
int current_brt = 0;
const int i_step = 4;
int dir = i_step;
unsigned long timestamp;
const unsigned long time_interval = 10;

void setup() {
  for ( int i = 0; i < sizeof(led_pins); i++ ) {
    pinMode( led_pins[i], OUTPUT );
  }

  timestamp = millis();
}

void loop() {
  unsigned long frame_ts = millis();

  if ( frame_ts - timestamp >= time_interval ) {
    timestamp = frame_ts;

    for ( int i = 0; i < sizeof(led_pins); i++ ) {
      analogWrite(led_pins[i], i % 2?current_brt:1023 - current_brt);
    }

    current_brt += dir;
    if( current_brt >= 1023 ){
      current_brt = 1023;
      dir = -i_step;
    }
    if( current_brt <= 0 ){
      current_brt = 0;
      dir = i_step;
    }
  }
}