--- a/servo/components/style/values/specified/length.rs
+++ b/servo/components/style/values/specified/length.rs
@@ -135,16 +135,19 @@ impl FontRelativeLength {
                     context.rule_cache_conditions.borrow_mut()
                         .set_font_size_dependency(
                             reference_font_size.into()
                         );
                 }
                 (reference_font_size, length)
             },
             FontRelativeLength::Ex(length) => {
+                if context.for_non_inherited_property.is_some() {
+                    context.rule_cache_conditions.borrow_mut().set_uncacheable();
+                }
                 let reference_size = match query_font_metrics(context, reference_font_size) {
                     FontMetricsQueryResult::Available(metrics) => {
                         metrics.x_height
                     },
                     // https://drafts.csswg.org/css-values/#ex
                     //
                     //     In the cases where it is impossible or impractical to
                     //     determine the x-height, a value of 0.5em must be
@@ -152,16 +155,19 @@ impl FontRelativeLength {
                     //
                     FontMetricsQueryResult::NotAvailable => {
                         reference_font_size.scale_by(0.5)
                     },
                 };
                 (reference_size, length)
             },
             FontRelativeLength::Ch(length) => {
+                if context.for_non_inherited_property.is_some() {
+                    context.rule_cache_conditions.borrow_mut().set_uncacheable();
+                }
                 let reference_size = match query_font_metrics(context, reference_font_size) {
                     FontMetricsQueryResult::Available(metrics) => {
                         metrics.zero_advance_measure
                     },
                     // https://drafts.csswg.org/css-values/#ch
                     //
                     //     In the cases where it is impossible or impractical to
                     //     determine the measure of the “0” glyph, it must be