*{
  box-sizing: border-box;
  font-family: Calibri;
}

body {
  margin: 0px;
}

.main_container {

  display:block;
  position: relative;
  
  background-color: rgb(255, 255, 255);
  margin: 0px auto 0px auto;
  height: 100dvh;
  width: 100vW;
  padding: 0px;
  overflow: hidden;

  /*border: solid 1px rgb(255 0 0);*/
  
}

.div_prosty{

  display: block;
  position: relative;
  width: 200px;
  height: 200px;
  background-color: rgb(226, 228, 138);
  border: 1px solid black;
  padding: 10px;

}

main-container:not(:defined){
  display: none;
 }

 /*

 to jest druga możliwość - standardowo opacity 0 , a gdy jest defined to 1

 main-container {
  opacity: 0;
}

 main-container:defined {
  opacity: 1;
}

*/