diff --git a/gfx/thebes/gfxFont.cpp b/gfx/thebes/gfxFont.cpp
index 2985cd58dfe..ec60d05dee9 100644
--- a/gfx/thebes/gfxFont.cpp
+++ b/gfx/thebes/gfxFont.cpp
@@ -44,6 +44,7 @@
 #include "gfxFontTest.h"
 
 #include "harfbuzz/hb.h"
+#include "harfbuzz/hb-ot.h"
 
 #include "nsCRT.h"
 #include "GeckoProfiler.h"
@@ -1492,6 +1493,54 @@ gfxFont::GetFontTable(uint32_t aTag) {
                                                 haveTable ? &buffer : nullptr);
 }
 
+static hb_blob_t *
+HBGetTable(hb_face_t *face, hb_tag_t aTag, void *aUserData)
+{
+    gfxFont* font = static_cast<gfxFont*>(aUserData);
+    return font->GetFontTable(aTag);
+}
+
+static bool
+HasLayoutFeatureInvolving(hb_face_t *aFace, hb_tag_t aTag, uint16_t aGlyph)
+{
+    hb_set_t *lookups = hb_set_create();
+    hb_set_t *glyphs = hb_set_create();
+
+    hb_ot_layout_collect_lookups(aFace, aTag, nullptr, nullptr, nullptr,
+                                 lookups);
+
+    hb_codepoint_t index = -1;
+    while (hb_set_next(lookups, &index)) {
+        hb_ot_layout_lookup_collect_glyphs(aFace, aTag, index,
+                                           glyphs, glyphs, glyphs,
+                                           glyphs);
+    }
+
+    bool result = hb_set_has(glyphs, aGlyph);
+
+    hb_set_destroy(glyphs);
+    hb_set_destroy(lookups);
+
+    return result;
+}
+
+bool
+gfxFont::CheckForFeaturesInvolvingSpace()
+{
+    hb_face_t *face = hb_face_create_for_tables(HBGetTable, this, nullptr);
+
+    bool result = (hb_ot_layout_has_substitution(face) &&
+                   HasLayoutFeatureInvolving(face, HB_OT_TAG_GSUB,
+                                             GetSpaceGlyph())) ||
+                  (hb_ot_layout_has_positioning(face) &&
+                   HasLayoutFeatureInvolving(face, HB_OT_TAG_GPOS,
+                                             GetSpaceGlyph()));
+
+    hb_face_destroy(face);
+
+    return result;
+}
+
 /**
  * A helper function in case we need to do any rounding or other
  * processing here.
@@ -2710,12 +2759,18 @@ gfxFont::SplitAndInitTextRun(gfxContext *aContext,
         return true;
     }
 
+    uint32_t flags = aTextRun->GetFlags();
+    if (BypassShapedWordCache(flags)) {
+        return ShapeTextWithoutWordCache(aContext, aString + aRunStart,
+                                         aRunStart, aRunLength, aRunScript,
+                                         aTextRun);
+    }
+
     InitWordCache();
 
     // the only flags we care about for ShapedWord construction/caching
-    uint32_t flags = aTextRun->GetFlags() &
-        (gfxTextRunFactory::TEXT_IS_RTL |
-         gfxTextRunFactory::TEXT_DISABLE_OPTIONAL_LIGATURES);
+    flags &= (gfxTextRunFactory::TEXT_IS_RTL |
+              gfxTextRunFactory::TEXT_DISABLE_OPTIONAL_LIGATURES);
     if (sizeof(T) == sizeof(uint8_t)) {
         flags |= gfxTextRunFactory::TEXT_IS_8BIT;
     }
diff --git a/gfx/thebes/gfxFont.h b/gfx/thebes/gfxFont.h
index 5685b919e72..bc944b93c33 100644
--- a/gfx/thebes/gfxFont.h
+++ b/gfx/thebes/gfxFont.h
@@ -216,6 +216,7 @@ public:
         mIgnoreGSUB(false),
         mSVGInitialized(false),
         mWeight(500), mStretch(NS_FONT_STRETCH_NORMAL),
+        mHasSpaceFeaturesInitialized(false),
         mCheckedForGraphiteTables(false),
         mHasCmapTable(false),
         mUVSOffset(0), mUVSData(nullptr),
@@ -348,6 +349,9 @@ public:
     uint16_t         mWeight;
     int16_t          mStretch;
 
+    bool             mHasSpaceFeatures;
+    bool             mHasSpaceFeaturesInitialized;
+
     bool             mHasGraphiteTables;
     bool             mCheckedForGraphiteTables;
     bool             mHasCmapTable;
@@ -379,6 +383,7 @@ protected:
         mIgnoreGSUB(false),
         mSVGInitialized(false),
         mWeight(500), mStretch(NS_FONT_STRETCH_NORMAL),
+        mHasSpaceFeaturesInitialized(false),
         mCheckedForGraphiteTables(false),
         mHasCmapTable(false),
         mUVSOffset(0), mUVSData(nullptr),
@@ -1561,6 +1566,26 @@ public:
     { return gfxPlatform::GetPlatform()->GetScaledFontForFont(aTarget, this); }
 
 protected:
+    bool BypassShapedWordCache(uint32_t aRunFlags) {
+        // When creating a textrun in optimizeSpeed mode, we always use the
+        // word cache (which means we do not support features that span
+        // inter-word spaces)
+        if (aRunFlags & gfxTextRunFactory::TEXT_OPTIMIZE_SPEED) {
+            return false;
+        }
+        // We record the presence of space-dependent features in the font entry
+        // so that subsequent instantiations for the same font face won't
+        // require us to re-check the tables; however, the actual check is done
+        // by gfxFont because not all font entry subclasses know how to create
+        // a harfbuzz face for introspection.
+        gfxFontEntry *fe = GetFontEntry();
+        if (!fe->mHasSpaceFeaturesInitialized) {
+            fe->mHasSpaceFeaturesInitialized = true;
+            fe->mHasSpaceFeatures = CheckForFeaturesInvolvingSpace();
+        }
+        return fe->mHasSpaceFeatures;
+    }
+
     // For 8-bit text, expand to 16-bit and then call the following method.
     bool ShapeText(gfxContext    *aContext,
                    const uint8_t *aText,
@@ -1617,6 +1642,8 @@ protected:
                                        int32_t     aScript,
                                        gfxTextRun *aTextRun);
 
+    bool CheckForFeaturesInvolvingSpace();
+
     nsRefPtr<gfxFontEntry> mFontEntry;
 
     struct CacheHashKey {
diff --git a/layout/media/symbols.def.in b/layout/media/symbols.def.in
index 7dfe2651cae..29ffdb28d4b 100644
--- a/layout/media/symbols.def.in
+++ b/layout/media/symbols.def.in
@@ -544,7 +544,15 @@ hb_font_set_funcs
 hb_font_set_ppem
 hb_font_set_scale
 hb_language_from_string
+hb_ot_layout_collect_lookups
+hb_ot_layout_has_positioning
+hb_ot_layout_has_substitution
+hb_ot_layout_lookup_collect_glyphs
 hb_ot_tag_to_language
+hb_set_create
+hb_set_destroy
+hb_set_has
+hb_set_next
 hb_shape
 hb_unicode_funcs_create
 hb_unicode_funcs_get_empty