diff --git a/content/canvas/src/nsCanvasRenderingContext2D.cpp b/content/canvas/src/nsCanvasRenderingContext2D.cpp
index e2169be5fc85..b969458853d1 100644
--- a/content/canvas/src/nsCanvasRenderingContext2D.cpp
+++ b/content/canvas/src/nsCanvasRenderingContext2D.cpp
@@ -1751,83 +1751,53 @@ nsCanvasRenderingContext2D::ArcTo(float x1, float y1, float x2, float y2, float
     if (!FloatValidate(x1,y1,x2,y2,radius))
         return NS_ERROR_DOM_SYNTAX_ERR;
 
-    if (radius <= 0)
+    if (radius < 0)
         return NS_ERROR_DOM_INDEX_SIZE_ERR;
 
-    /* This is an adaptation of the cairo_arc_to patch from Behdad
-     * Esfahbod; once that patch is accepted, we should remove this
-     * and just call cairo_arc_to() directly.
-     */
-    
-    double angle0, angle1, angle2, angled;
-    double d0, d2;
-    double sin_, cos_;
-    double dc;
-    int forward;
-
     gfxPoint p0 = mThebes->CurrentPoint();
 
-    angle0 = atan2 (p0.y - y1, p0.x - x1); /* angle from (x1,y1) to (p0.x,p0.y) */
-    angle2 = atan2 (y2 - y1, x2 - x1); /* angle from (x1,y1) to (x2,y2) */
-    angle1 = (angle0 + angle2) / 2;    /* angle from (x1,y1) to (xc,yc) */
+    double dir, a2, b2, c2, cosx, sinx, d, anx, any, bnx, bny, x3, y3, x4, y4, cx, cy, angle0, angle1;
+    bool anticlockwise;
 
-    angled = angle2 - angle0;          /* the angle (p0.x,p0.y)--(x1,y1)--(x2,y2) */
-
-    /* Shall we go forward or backward? */
-    if (angled > M_PI || (angled < 0 && angled > -M_PI)) {
-        angle1 += M_PI;
-        angled = 2 * M_PI - angled;
-        forward = 1;
-    } else {
-        double tmp;
-        tmp = angle0;
-        angle0 = angle2;
-        angle2 = tmp;
-        forward = 0;
+    if ((x1 == p0.x && y1 == p0.y) || (x1 == x2 && y1 == y2) || radius == 0) {
+        mThebes->LineTo(gfxPoint(x1, y1));
+        return NS_OK;
     }
 
-    angle0 += M_PI_2; /* angle from (xc,yc) to (p0.x,p0.y) */
-    angle2 -= M_PI_2; /* angle from (xc,yc) to (x2,y2) */
-    angled /= 2;      /* the angle (p0.x,p0.y)--(x1,y1)--(xc,yc) */
-
-
-    /* distance from (x1,y1) to (p0.x,p0.y) */
-    d0 = sqrt ((p0.x-x1)*(p0.x-x1)+(p0.y-y1)*(p0.y-y1));
-    /* distance from (x2,y2) to (p0.x,p0.y) */
-    d2 = sqrt ((x2-x1)*(x2-x1)+(y2-y1)*(y2-y1));
-
-    dc = -1;
-    sin_ = sin(angled);
-    cos_ = cos(angled);
-    if (fabs(cos_) >= 1e-5) { /* the arc may not fit */
-        /* min distance of end-points from corner */
-        double min_d = d0 < d2 ? d0 : d2;
-        /* max radius of an arc that fits */
-        double max_r = min_d * sin_ / cos_;
-
-        if (radius > max_r) {
-            /* arc with requested radius doesn't fit */
-            radius = (float) max_r;
-            dc = min_d / cos_; /* distance of (xc,yc) from (x1,y1) */
-        }
+    dir = (x2-x1)*(p0.y-y1) + (y2-y1)*(x1-p0.x);
+    if (dir == 0) {
+        mThebes->LineTo(gfxPoint(x1, y1));
+        return NS_OK;
     }
 
-    if (dc < 0)
-        dc = radius / sin_; /* distance of (xc,yc) from (x1,y1) */
+    a2 = (p0.x-x1)*(p0.x-x1) + (p0.y-y1)*(p0.y-y1);
+    b2 = (x1-x2)*(x1-x2) + (y1-y2)*(y1-y2);
+    c2 = (p0.x-x2)*(p0.x-x2) + (p0.y-y2)*(p0.y-y2);
+    cosx = (a2+b2-c2)/(2*sqrt(a2*b2));
 
+    sinx = sqrt(1 - cosx*cosx);
+    d = radius / ((1 - cosx) / sinx);
 
-    /* find (cx,cy), the center of the arc */
-    gfxPoint c(x1 + sin(angle1) * dc, y1 + cos(angle1) * dc);
+    anx = (x1-p0.x) / sqrt(a2);
+    any = (y1-p0.y) / sqrt(a2);
+    bnx = (x1-x2) / sqrt(b2);
+    bny = (y1-y2) / sqrt(b2);
+    x3 = x1 - anx*d;
+    y3 = y1 - any*d;
+    x4 = x1 - bnx*d;
+    y4 = y1 - bny*d;
+    anticlockwise = (dir < 0);
+    cx = x3 + any*radius*(anticlockwise ? 1 : -1);
+    cy = y3 - anx*radius*(anticlockwise ? 1 : -1);
+    angle0 = atan2((y3-cy), (x3-cx));
+    angle1 = atan2((y4-cy), (x4-cx));
 
-    /* the arc operation draws the line from current point (p0.x,p0.y)
-     * to arc center too. */
+    mThebes->LineTo(gfxPoint(x3, y3));
 
-    if (forward)
-        mThebes->Arc(c, radius, angle0, angle2);
+    if (anticlockwise)
+        mThebes->NegativeArc(gfxPoint(cx, cy), radius, angle0, angle1);
     else
-        mThebes->NegativeArc(c, radius, angle2, angle0);
-
-    mThebes->LineTo(gfxPoint(x2, y2));
+        mThebes->Arc(gfxPoint(cx, cy), radius, angle0, angle1);
 
     return NS_OK;
 }
diff --git a/content/canvas/test/test_2d.path.arcTo.coincide.1.html b/content/canvas/test/test_2d.path.arcTo.coincide.1.html
index 652f0b6ef135..cf2789dcad11 100644
--- a/content/canvas/test/test_2d.path.arcTo.coincide.1.html
+++ b/content/canvas/test/test_2d.path.arcTo.coincide.1.html
@@ -55,9 +55,9 @@ ctx.moveTo(50, 25);
 ctx.arcTo(50, 25, 100, 25, 1);
 ctx.stroke();
 
-todo_isPixel(ctx, 50,1, 0,255,0,255, "50,1", "0,255,0,255", 0);
-todo_isPixel(ctx, 50,25, 0,255,0,255, "50,25", "0,255,0,255", 0);
-todo_isPixel(ctx, 50,48, 0,255,0,255, "50,48", "0,255,0,255", 0);
+isPixel(ctx, 50,1, 0,255,0,255, "50,1", "0,255,0,255", 0);
+isPixel(ctx, 50,25, 0,255,0,255, "50,25", "0,255,0,255", 0);
+isPixel(ctx, 50,48, 0,255,0,255, "50,48", "0,255,0,255", 0);
 
 SimpleTest.finish();
 
diff --git a/content/canvas/test/test_2d.path.arcTo.collinear.1.html b/content/canvas/test/test_2d.path.arcTo.collinear.1.html
index 142e707d5dbd..ce1b5b0bdc51 100644
--- a/content/canvas/test/test_2d.path.arcTo.collinear.1.html
+++ b/content/canvas/test/test_2d.path.arcTo.collinear.1.html
@@ -54,7 +54,7 @@ ctx.moveTo(-100, 25);
 ctx.arcTo(0, 25, 100, 25, 1);
 ctx.stroke();
 
-todo_isPixel(ctx, 50,25, 0,255,0,255, "50,25", "0,255,0,255", 0);
+isPixel(ctx, 50,25, 0,255,0,255, "50,25", "0,255,0,255", 0);
 
 SimpleTest.finish();
 
diff --git a/content/canvas/test/test_2d.path.arcTo.shape.curve1.html b/content/canvas/test/test_2d.path.arcTo.shape.curve1.html
index 872331cf9fbb..d345690eea17 100644
--- a/content/canvas/test/test_2d.path.arcTo.shape.curve1.html
+++ b/content/canvas/test/test_2d.path.arcTo.shape.curve1.html
@@ -69,8 +69,8 @@ isPixel(ctx, 73,27, 0,255,0,255, "73,27", "0,255,0,255", 0);
 isPixel(ctx, 78,36, 0,255,0,255, "78,36", "0,255,0,255", 0);
 isPixel(ctx, 79,35, 0,255,0,255, "79,35", "0,255,0,255", 0);
 isPixel(ctx, 80,44, 0,255,0,255, "80,44", "0,255,0,255", 0);
-todo_isPixel(ctx, 80,45, 0,255,0,255, "80,45", "0,255,0,255", 0);
-todo_isPixel(ctx, 80,46, 0,255,0,255, "80,46", "0,255,0,255", 0);
+isPixel(ctx, 80,45, 0,255,0,255, "80,45", "0,255,0,255", 0);
+isPixel(ctx, 80,46, 0,255,0,255, "80,46", "0,255,0,255", 0);
 
 SimpleTest.finish();
 
diff --git a/content/canvas/test/test_2d.path.arcTo.shape.curve2.html b/content/canvas/test/test_2d.path.arcTo.shape.curve2.html
index bac4f5707d1a..94bdb716cb85 100644
--- a/content/canvas/test/test_2d.path.arcTo.shape.curve2.html
+++ b/content/canvas/test/test_2d.path.arcTo.shape.curve2.html
@@ -58,13 +58,13 @@ ctx.moveTo(10, 25);
 ctx.arcTo(75, 25, 75, 60, 20);
 ctx.stroke();
 
-todo_isPixel(ctx, 50,25, 0,255,0,255, "50,25", "0,255,0,255", 0);
+isPixel(ctx, 50,25, 0,255,0,255, "50,25", "0,255,0,255", 0);
 isPixel(ctx, 55,19, 0,255,0,255, "55,19", "0,255,0,255", 0);
 isPixel(ctx, 55,20, 0,255,0,255, "55,20", "0,255,0,255", 0);
-todo_isPixel(ctx, 55,21, 0,255,0,255, "55,21", "0,255,0,255", 0);
+isPixel(ctx, 55,21, 0,255,0,255, "55,21", "0,255,0,255", 0);
 isPixel(ctx, 64,22, 0,255,0,255, "64,22", "0,255,0,255", 0);
 isPixel(ctx, 65,21, 0,255,0,255, "65,21", "0,255,0,255", 0);
-todo_isPixel(ctx, 72,28, 0,255,0,255, "72,28", "0,255,0,255", 0);
+isPixel(ctx, 72,28, 0,255,0,255, "72,28", "0,255,0,255", 0);
 isPixel(ctx, 73,27, 0,255,0,255, "73,27", "0,255,0,255", 0);
 isPixel(ctx, 78,36, 0,255,0,255, "78,36", "0,255,0,255", 0);
 isPixel(ctx, 79,35, 0,255,0,255, "79,35", "0,255,0,255", 0);
diff --git a/content/canvas/test/test_2d.path.arcTo.shape.end.html b/content/canvas/test/test_2d.path.arcTo.shape.end.html
index cc5f42812c9e..5f679ef599ac 100644
--- a/content/canvas/test/test_2d.path.arcTo.shape.end.html
+++ b/content/canvas/test/test_2d.path.arcTo.shape.end.html
@@ -48,10 +48,10 @@ ctx.arcTo(-100, 25, 200, 25, 10);
 ctx.stroke();
 
 isPixel(ctx, 1,1, 0,255,0,255, "1,1", "0,255,0,255", 0);
-todo_isPixel(ctx, 1,48, 0,255,0,255, "1,48", "0,255,0,255", 0);
-todo_isPixel(ctx, 50,25, 0,255,0,255, "50,25", "0,255,0,255", 0);
+isPixel(ctx, 1,48, 0,255,0,255, "1,48", "0,255,0,255", 0);
+isPixel(ctx, 50,25, 0,255,0,255, "50,25", "0,255,0,255", 0);
 isPixel(ctx, 98,1, 0,255,0,255, "98,1", "0,255,0,255", 0);
-todo_isPixel(ctx, 98,48, 0,255,0,255, "98,48", "0,255,0,255", 0);
+isPixel(ctx, 98,48, 0,255,0,255, "98,48", "0,255,0,255", 0);
 
 SimpleTest.finish();
 
diff --git a/content/canvas/test/test_2d.path.arcTo.shape.start.html b/content/canvas/test/test_2d.path.arcTo.shape.start.html
index 23a7bc172d0b..4a6aa40cb2a9 100644
--- a/content/canvas/test/test_2d.path.arcTo.shape.start.html
+++ b/content/canvas/test/test_2d.path.arcTo.shape.start.html
@@ -48,10 +48,10 @@ ctx.arcTo(200, 25, 200, 50, 10);
 ctx.stroke();
 
 isPixel(ctx, 1,1, 0,255,0,255, "1,1", "0,255,0,255", 0);
-todo_isPixel(ctx, 1,48, 0,255,0,255, "1,48", "0,255,0,255", 0);
+isPixel(ctx, 1,48, 0,255,0,255, "1,48", "0,255,0,255", 0);
 isPixel(ctx, 50,25, 0,255,0,255, "50,25", "0,255,0,255", 0);
 isPixel(ctx, 98,1, 0,255,0,255, "98,1", "0,255,0,255", 0);
-todo_isPixel(ctx, 98,48, 0,255,0,255, "98,48", "0,255,0,255", 0);
+isPixel(ctx, 98,48, 0,255,0,255, "98,48", "0,255,0,255", 0);
 
 SimpleTest.finish();
 
diff --git a/content/canvas/test/test_2d.path.arcTo.zero.1.html b/content/canvas/test/test_2d.path.arcTo.zero.1.html
index 4bde14652512..c4e63271c6c1 100644
--- a/content/canvas/test/test_2d.path.arcTo.zero.1.html
+++ b/content/canvas/test/test_2d.path.arcTo.zero.1.html
@@ -50,7 +50,7 @@ isPixel(ctx, 50,25, 0,255,0,255, "50,25", "0,255,0,255", 0);
 } catch (e) {
     _thrown_outer = true;
 }
-todo(!_thrown_outer, 'should not throw exception');
+ok(!_thrown_outer, 'should not throw exception');
 
 SimpleTest.finish();
 
diff --git a/content/canvas/test/test_2d.path.arcTo.zero.2.html b/content/canvas/test/test_2d.path.arcTo.zero.2.html
index 294bdfcb1905..1647a06be5fb 100644
--- a/content/canvas/test/test_2d.path.arcTo.zero.2.html
+++ b/content/canvas/test/test_2d.path.arcTo.zero.2.html
@@ -50,7 +50,7 @@ isPixel(ctx, 50,25, 0,255,0,255, "50,25", "0,255,0,255", 0);
 } catch (e) {
     _thrown_outer = true;
 }
-todo(!_thrown_outer, 'should not throw exception');
+ok(!_thrown_outer, 'should not throw exception');
 
 SimpleTest.finish();