diff --git a/spec/styles-element-spec.coffee b/spec/styles-element-spec.coffee
index ba98633e5..2182594f0 100644
--- a/spec/styles-element-spec.coffee
+++ b/spec/styles-element-spec.coffee
@@ -94,3 +94,14 @@ describe "StylesElement", ->
       expect(element.firstChild.sheet.cssRules[0].selectorText).toBe ':host'
       expect(element.firstChild.sheet.cssRules[1].selectorText).toBe ':host(.mini)'
       expect(element.firstChild.sheet.cssRules[2].selectorText).toBe ':host(:focus)'
+
+    it "defers selector upgrade until the element is attached", ->
+      element = new StylesElement
+      element.setAttribute('context', 'atom-text-editor')
+      element.initialize()
+
+      atom.styles.addStyleSheet ".editor {background: black;}", context: 'atom-text-editor'
+      expect(element.firstChild.sheet).toBeNull()
+
+      document.querySelector('#jasmine-content').appendChild(element)
+      expect(element.firstChild.sheet.cssRules[0].selectorText).toBe ':host'
diff --git a/src/styles-element.coffee b/src/styles-element.coffee
index 5bb856209..b011113fc 100644
--- a/src/styles-element.coffee
+++ b/src/styles-element.coffee
@@ -18,6 +18,9 @@ class StylesElement extends HTMLElement
     @styleElementClonesByOriginalElement = new WeakMap
 
   attachedCallback: ->
+    if @context is 'atom-text-editor'
+      for styleElement in @children
+        @upgradeDeprecatedSelectors(styleElement)
     @initialize()
 
   detachedCallback: ->
@@ -48,6 +51,7 @@ class StylesElement extends HTMLElement
     return unless @styleElementMatchesContext(styleElement)
 
     styleElementClone = styleElement.cloneNode(true)
+    styleElementClone.sourcePath = styleElement.sourcePath
     styleElementClone.context = styleElement.context
     @styleElementClonesByOriginalElement.set(styleElement, styleElementClone)
 
@@ -61,7 +65,7 @@ class StylesElement extends HTMLElement
     @insertBefore(styleElementClone, insertBefore)
 
     if @context is 'atom-text-editor'
-      @upgradeDeprecatedSelectors(styleElementClone, styleElement.sourcePath)
+      @upgradeDeprecatedSelectors(styleElementClone)
 
     @emitter.emit 'did-add-style-element', styleElementClone
 
@@ -82,7 +86,9 @@ class StylesElement extends HTMLElement
   styleElementMatchesContext: (styleElement) ->
     not @context? or styleElement.context is @context
 
-  upgradeDeprecatedSelectors: (styleElement, sourcePath) ->
+  upgradeDeprecatedSelectors: (styleElement) ->
+    return unless styleElement.sheet?
+
     upgradedSelectors = []
 
     for rule in styleElement.sheet.cssRules
@@ -99,7 +105,7 @@ class StylesElement extends HTMLElement
         upgradedSelectors.push({inputSelector, outputSelector})
 
     if upgradedSelectors.length > 0
-      warning = "Upgraded the following syntax theme selectors in `#{sourcePath}` for shadow DOM compatibility:\n\n"
+      warning = "Upgraded the following syntax theme selectors in `#{styleElement.sourcePath}` for shadow DOM compatibility:\n\n"
       for {inputSelector, outputSelector} in upgradedSelectors
         warning += "`#{inputSelector}` => `#{outputSelector}`\n"