Font weight is thicker on windows than mac?

Question

I don't know whats wrong with my code? The font weight changes when I'm on windows, but it stays the same on my android and mac. I've also attempted to change the font face because I that it might have been that windows doesn't come installed with the helvetica font.

@font-face {
 font-family: Helvetica;
  src: url(Helvetica.dfont);
}
a,p{
 text-decoration: none;
 font-weight: normal;
 font-family: Helvetica;
 font-size: 12px;
}
li{
 list-style-type: georgian;
 font-family: Helvetica;
 font-weight: lighter;
 padding-top: 5px;
}
h3{
 font-weight: lighter;
 font-family: Helvetica;
 margin: 0px;
}
hr{
 height: 2px;
 background-color: lightgray;
 border: 0px;
}
/*MOBILE SUPPORT*/
@media screen and (max-width: 414px) and (min-width: 337px) {
 #headnav{
  text-align: center;
  margin-right: 40px;
 }
 #updatelog{
  margin-top: 10px;
 }

 img.footer_pi{
  width: 90%;
  height: 90%;
 }
}

/*END OF MOBILE SUPPORT*/
/*wrappers*/
.wrapper{
 margin: 0 auto;
 padding: 10px;
 display: block;
 max-width: 750px;
}

footer.wrapper{
 overflow: hidden;
}
/*end of wrappers*/

/*homepage*/
#updatelog{
 margin-top: 10px;
 margin-left: 5px;
}
#robotics_notes{
 max-width: 100%;
 width: auto;
}

article#home_article > img{
 float: left;
}

article#home_article > p{
 padding-top:10px;
 clear: both;
}
article#home_article > aside{
 display: inline-block;
}

#homepage_header{
 font-family: Helvetica;
 font-weight: 300;
} 
#selected{
 background-color: #65448C;
}

div>h1{
 margin: 0 auto;
 text-align: center;
}

li > a{
 color: #ED1148;
 padding: 8px;
}
ul#headnav > li{
 list-style-type: none;
 display: inline-block;
}
li > a:hover{
 background-color: #ED1148;
}
li:hover a {
 color: white;
}
/*end of homepage*/
/*login*/
#middle{
 background-color: #E6CCCC;
 padding-top: 30px;
 width: 25%;
 min-width: 249px;
}
.login{
  text-align: center;
}

button.login{
 font-family: Helvetica;
 margin-top: 10px;
 margin-bottom: 7px;
}
h2.login{
 color: #6A1B1B;
 display:block;
 padding: 0px;
 margin-top: 0px;
}
/*end of login*/
/*footer*/
 a.footer_pi{
  width: auto;
  max-width: 100%;
  visibility: visible; 
  float: right;
 }
 img.footer_pi{
  opacity: 0.65;
  transition: opacity .5s ease-in;
 }
 img:hover{
  opacity: 1;
 }
/*end of footer*/

http://intimite.biz.tm/