Sift together 1/3 cup sugar, flour, and salt. Stir in the rind, and set aside. Beat egg whites (at room temperature) and cream of tartar at high speed of a mixer until foamy. Add the remaining sugar, 1 tablespoon at a time, beating until stiff peaks form. Fold in the vanilla. Sprinkle flour mixture over egg white mixture, and fold in gently.
Spoon batter into an ungreased 9 x 5-inch loaf pan, spreading evenly. Break large air pockets by cutting through batter with a knife. Bake at 350° for 30 minutes or until cake springs back when lightly touched. Invert pan on a wire rack; let cool completely. Loosen cake from sides of pan, using a narrow spatula; remove from pan.
Place strawberries and syrup in food processor, and process until smooth. Pour into a bowl; stir in the chopped grapefruit, and serve with cake.