I especially like the feature which lets you INSERT a body in a simulation.  
As I understood the html version then contacts the horizons jpl website and retrieves the x,y,z,vx,vy,vz coordinates of the specified body and uses them in the simulation . Fantastic .  
http://ssd.jpl.nasa.gov/?horizons_doc#specific_quantities
Is it possible to clarify Tony which settings you use for retrieving the data ?  
( Ie : reference frame choosen (JC , FK ) and the options concerning the light -time ) .  
The latter may be important in case of far away bodies such as Pluto where the light time comes yes/no into play depending upon ...

Quote from Tony on 08/09/15 at 09:13:24:

Thanks Tony for the link .  
Concerning the light time correction : It is assumed gravity works instantanious , just as Newton derived .  This means the action of far away bodies comes from another position as to where we "see" the body . Just wondered with which value GravSim works . Seems ok  
Edit : maybe the "light time correction " may give some funny results if one would observe eclipses , such as solar eclipse in which the observed time may be off by several minutes .    

The camera position and "look at" feature was done with additional coding.  The crescents were done with additional coding as well.
There are some libraries available to HTML5 such as WebGL and three.js.  I've played around with each a little, but not all browsers support them, so I didn't use them.
In the future, I might use them to include surfaces of planets, so when you look at Earth, you see continents and oceans instead of a big blue circle.
 
Crescents were done by computing the phase angle between the camera/object and object/sun lines.  The shape of the terminator line is a half ellipse, alternating between e = 0 (circle) and e = 1, straight line.  In the code, look for the function: function drawCrescentA()

Yes, the drawing phases is done by drawCrescentA().  But lots of math is done outside that function before it is called.
You need to know the vectors for the various objects.
 
The two vectors are:
vector1: the line connecting the target object to the Sun
vector2: the line connecting the target object to the camera object
 
So in the "Moon occults Venus" sim when it draws the Moon, vector 1 is the line connecting the Moon to the Sun and vector2 is the line connecting the Moon to the Earth.
When it draws Venus, vector1 is the line connecting Venus to the Sun, and vector2 is the line connecting Venus to the Earth.
 
The following code gets executed just prior to calling drawCrescentA();
 
These 2 lines of code create the vectors in heliocentric coordinates (Sun is at 0, 0, 0)
 
vector1x = objx[k]; vector1y = objy[k]; vector1z = objz[k];
vector2x = objx[k] - objx[fromObjA]; vector2y = objy[k] - objy[fromObjA]; vector2z = objz[k] - objz[fromObjA];
 
 
These 2 lines use Pythagorean theorem to get the lengths or magnitudes of the vectors
 
vector1mag = Math.sqrt(vector1x * vector1x+ vector1y * vector1y + vector1z*vector1z);
vector2mag = Math.sqrt(vector2x * vector2x+ vector2y * vector2y + vector2z*vector2z);
 
 
To get the phase angle between 2 vectors, you use:
arccosine (dot product of the 2 vectors divided by the product of their magnitudes)
 
These two lines compute the dot product and the magnitude product
 
dot = vector1x * vector2x  + vector1y * vector2y  + vector1z * vector2z ;
mag = vector1mag * vector2mag;
 
 
Now you can compute the phase angle
 
theta = Math.acos(dot / mag);
 
 
Now you have to compute the orientation of the terminator line.  The terminator touches the limb of the planet on 2 points, 180 degrees apart.  This forms a line.
Find the slope of this line using slope = rise / run.
Then use arctangent to get the angle of that line.
 
 
rise = sY - sunY; run = sX - sunX; phi = Math.atan2(rise, run) - 0.5 * Math.PI;
 
 
Prevent quadrant error.
 
if (sunTheta > 90) {phi = phi - Math.PI}
 
 
Send to the drawCrescent function the index of the object to be drawn (k), plus its position on the screen (sX, sY), its angular size (plotsize), the phase angle (theta), and the orientation (phi).
plotsize, sX and sY were computed a few lines before the crescent code since they're the same whether the user chooses to display crescents or not.
 
drawCrescentA(k, sX, sY, plotsize, theta, phi);
 
The function drawCrescent takes this info and:
1. Draws a semi circle representing the dark limb of the object.
2. A semi ellipse representing the terminator (day/night line).
3. Colors this closed shape with the object's shadow color.
4. Draws another semi circle representing the bright limb of the object.
5. Redraws the same semi ellipse representing the terminator.
6. Colors this closed shape with the object's color.

Just one word to above posts Tony : waw !  
Great job !  
 
Edit : thought the DrawCrescent() was a standard function in html , but after looking at the code I see the function is coded as the parts you describe in the previous post . Nice !  
Second Edit : any chance the code may be extended with the anaglyph viewing function ? Would be nice to get some depth in the screen . I see parallel vieuwing is already inplemented . The step to anaglyph may therfor be possible .