There are several ways to reinforce a leg-rail or post-rail joint. The first is to use a single block that is vertically-oriented. This is often found at the post-rail joint. To avoid having to notch around the inside corner of a leg, two blocks are used instead. The first is glued to the front seat rail and flushed to the leg where the side rail meets. Then the second block is fit to the side rail and leg-block surface. Vertically-oriented blocks are typically just glued.
Another type of block is a diagonal brace whose grain is parallel to the rails. If they are the same width as the rail, they should be screwed since the glue surface is a mitred end (and less than 45 degrees on a trapezoidal seat). I've also seen diagonal braces that are dovetailed into the top edges of the seat rails. In this case, they are not as wide as the rails (1"W+/-) and are glued. There are undoubtedly other types of corner brace. These are the ones I am most familiar with.
The early method used was this? One brace is placed at the corner of each leg joint; to equalize the strain generated by the seat webbing, let me explain further? Lets take the front rail, leg, and the connecting side rail, as a example, a brace, say 5" long x 1"x1", this is placed equally across the corner, then marked with a pencil line, a housing joint is then made 1/4" deep in to the front rail & side rail.
the chair is then glued up.
After cleaning of the excess glue from all the joints, the proposed brace is placed over the precut housing joints and marked and cut to the corresponding housing joint, (I like to wedge this cut on brace) this is then just glued in place. When dry, and the webbing are stretched to the desired tension.
This Conclusion can now be made. No pressure is on the leg mortise & tenon? But on each rail housing only, the front pressure is transferred through to the side rail, and visa versa. So no other screw or nail is required, just a good tight fit, and glue.
If more info is required: if one see's my ribbon-back chair in the gallery, there is metal corner brackets in view at each corner, these are to aid transporting this chair from show to show only, and will be replaced with the timber ones as described above, when I finally need to finish it off.