engine_spx2html: avoid emitting negative padding values, which are illegal

Author: pkgw

crates/engine_spx2html/src/emission.rs

@@ -1172,14 +1172,20 @@ impl EmittingState {
 
         let element = self.create_elem(element, true, common);
 
+        // Negative padding values are illegal.
+        let pad_left = match -x_min_tex as f32 * self.rems_per_tex {
+            pl if pl <= 0.0 => 0.0,
+            pl => pl,
+        };
+
         write!(
             self.content,
             "<{} class=\"canvas {}\" style=\"width: {}rem; height: {}rem; padding-left: {}rem{}\">",
             element.name(),
             layout_class,
             (x_max_tex - x_min_tex) as f32 * self.rems_per_tex,
             (y_max_tex - y_min_tex) as f32 * self.rems_per_tex,
-            -x_min_tex as f32 * self.rems_per_tex,
+            pad_left,
             valign,
         )
         .unwrap();