Permite solo un punto y un decimal

onedecimal.html

<input type="text" onkeypress="return filterFloat(event,this);">

<script>
  function filterFloat(evt,input){
      // Backspace = 8, Enter = 13, ‘0′ = 48, ‘9′ = 57, ‘.’ = 46, ‘-’ = 43
      var key = window.Event ? evt.which : evt.keyCode;    
      var chark = String.fromCharCode(key);
      var tempValue = input.value+chark;
      if(key >= 48 && key <= 57){
          if(filter(tempValue)=== false){
              return false;
          }else{       
              return true;
          }
      }else{
            if(key == 8 || key == 13 || key == 0) {     
                return true;              
            }else if(key == 46){
                  if(filter(tempValue)=== false){
                      return false;
                  }else{       
                      return true;
                  }
            }else{
                return false;
            }
      }
  }
  function filter(__val__){
      var preg = /^([1-7]\.?[0-9]{0,1})$/; 
      if(preg.test(__val__) === true){
          return true;
      }else{
        return false;
      }
      
  }
</script>