BODY {
	font-family: Tahoma, Arial CE,Arial,Sans-serif;
	background-color: #F8F3D4;
	text-align: justify;
}

div.main {
}

div.article {
  float: center;
  width: 900px; 
}

Use a media query to add a breakpoint at 900px:
@media screen and (max-width: 930px) {
  div.article {
    width: 100%; /* The width is 100%, when the viewport is 900px or smaller */
  }
}

P {
  margin-left: 3pt;
  margin-right: 3pt;
  margin-top: 5pt;
  margin-bottom: 5pt;
  text-indent:1cm;
}

P.NoIndent {
  text-indent:0px;
}

H1 {
	font-size: 130%;
	font-weight: bold;
	background-color : #BCA887;
	padding-top: 3pt;
	padding-bottom: 3pt;
	margin-top: 5pt;
	margin-bottom: 16pt;
	color: black;
	text-align : center;
}

H2 {
  font-weight: bold;
  font-size: 110%;
  margin-top: 5pt;
  margin-bottom: 16pt;
  color: black;
}

H3 {
  font-weight: bold;
  font-size: 110%;
  color: black;
  margin-top: 5pt;
  margin-bottom: 16pt;
}

TH{
	padding-left: 3pt;
	padding-right: 3pt;
	padding-top: 1pt;
	padding-bottom: 1pt;
	background-color : #BCA887;
}

TD {
	padding-left: 3pt;
	padding-right: 3pt;
	padding-top: 1pt;
	padding-bottom: 1pt;
	background-color : #E7E2C0;
}

@media screen and (max-width: 500px) {
  TH {
    font-size: 80%; /* The font is 80%, when the viewport is 500px or smaller */
  }
  TD {
    font-size: 80%; /* The font is 80%, when the viewport is 500px or smaller */
  }
}


TD.H {
	padding-left: 3pt;
	padding-right: 3pt;
	padding-top: 1pt;
	padding-bottom: 1pt;
	background-color : #DCD7A3;
}

TD.clear {
  padding-left: 3pt;
  padding-right: 3pt;
  padding-top: 1pt;
  padding-bottom: 1pt;
  background : transparent;
}

TD.NoBorder {
  padding-left: 0pt;
  padding-right: 0pt;
  padding-top: 0pt;
  padding-bottom: 0pt;
  background : transparent;
}

TD.red {
  padding-left: 3pt;
  padding-right: 3pt;
  padding-top: 1pt;
  padding-bottom: 1pt;
  background-color : #FF6464;
}

TD.Photo {
  padding-left: 0pt;
  padding-right: 0pt;
  padding-top: 0pt;
  padding-bottom: 0pt;
  background-color : #ECECEC;
}

/* LI { */	/* tohle prasí menu v 3D !!! */
  /* margin-left: 3pt; */
  /* margin-right: 3pt; */
  /* margin-top: 5pt; */
  /* margin-bottom: 5pt; */
/* } */