Most OpenSCAD code builds the same shape here. The differences come from one choice: ReDo.Core builds exact solids, where OpenSCAD builds meshes of triangles.

Circles are exact

• $fn starts at 0, which means a true circle. circle(), cylinder() and rotate_extrude() make real curves, so there is no need to raise $fn to smooth a hole. Set $fn only when you want a polygon: $fn = 6 makes a hexagon.
• $fn above 512 is refused. No part needs that many sides, since the circle is already exact.
• $fa and $fs do nothing.
• sphere() is always exact, whatever $fn says.

What builds differently

• cylinder(h, r1) with no r2 is a straight cylinder. OpenSCAD tapers it to r2 = 1.
• + joins strings: "hole_" + 3 is "hole_3". OpenSCAD gives undef.
• A 2D shape keeps its height. OpenSCAD flattens a 2D shape onto the floor; here translate([0, 0, 2]) square(5) stays 2 mm up. The extrudes and hull() refuse a 2D shape that is not at z = 0.
• A 2D shape left in a part is part of it: a flat sheet with no thickness, such as a label or a pad on a board. OpenSCAD drops 2D shapes that sit beside 3D ones.
• A module's children are built before the module runs. A $ setting the module sets inside itself does not reach the children it was handed. One set on the call, m($fn = 6) { ... }, does.
• A module is built once for the same arguments, so an echo() inside it prints once for each different call.
• hull() samples curved faces. A hull of boxes is exact; a hull of round shapes is a solid of many small flat faces that stays within 0.1 % of the true one.
• offset() works on 2D shapes only, and keeps arcs as arcs.
• text() has one font, ReDo Sans (Liberation Sans, the font OpenSCAD draws by default), and lays text out as OpenSCAD does.
• render() does nothing: every shape is already exact.

What it refuses that OpenSCAD lets pass

Each of these stops the build with a message saying where, rather than building something you did not mean:

• a name nothing defines (OpenSCAD warns and reads undef);
• a name read before it is assigned, as in a = b; b = 2;;
• a named argument the call does not take;
• a 2D size given to cube();
• a polygon() whose outline crosses itself;
• a linear_extrude() with a twist, or a scale other than 1;
• a 2D shape in a difference() cutting a 3D one;
• a text() in another font, written right to left, in a script other than Latin, or with a character the font lacks;
• use <file>: use include instead;
• the commands listed under Refused on OpenSCAD support.

Limits on one part

So that a mistake cannot freeze your browser, one part may take:

Errors

An error names its line and column, and the library file when it is in one: line 2, column 8: expected ")", found ";".