diff --git a/package.json b/package.json
index 63b00ee8f..3a33472f3 100644
--- a/package.json
+++ b/package.json
@@ -45,7 +45,8 @@
     "nslog": "^3",
     "oniguruma": "6.1.0",
     "pathwatcher": "~6.5",
-    "postcss-selector-parser": "^2.2.1",
+    "postcss": "5.2.4",
+    "postcss-selector-parser": "2.2.1",
     "property-accessors": "^1.1.3",
     "random-words": "0.0.1",
     "resolve": "^1.1.6",
diff --git a/spec/style-manager-spec.js b/spec/style-manager-spec.js
index 19451ffea..65fc24e9a 100644
--- a/spec/style-manager-spec.js
+++ b/spec/style-manager-spec.js
@@ -1,39 +1,107 @@
 const StyleManager = require('../src/style-manager')
 
 describe('StyleManager', () => {
-  let [manager, addEvents, removeEvents, updateEvents] = []
+  let [styleManager, addEvents, removeEvents, updateEvents] = []
 
   beforeEach(() => {
-    manager = new StyleManager({configDirPath: atom.getConfigDirPath()})
+    styleManager = new StyleManager({configDirPath: atom.getConfigDirPath()})
     addEvents = []
     removeEvents = []
     updateEvents = []
-    manager.onDidAddStyleElement((event) => { addEvents.push(event) })
-    manager.onDidRemoveStyleElement((event) => { removeEvents.push(event) })
-    manager.onDidUpdateStyleElement((event) => { updateEvents.push(event) })
+    styleManager.onDidAddStyleElement((event) => { addEvents.push(event) })
+    styleManager.onDidRemoveStyleElement((event) => { removeEvents.push(event) })
+    styleManager.onDidUpdateStyleElement((event) => { updateEvents.push(event) })
   })
 
   describe('::addStyleSheet(source, params)', () => {
     it('adds a stylesheet based on the given source and returns a disposable allowing it to be removed', () => {
-      const disposable = manager.addStyleSheet('a {color: red}')
+      const disposable = styleManager.addStyleSheet('a {color: red}')
       expect(addEvents.length).toBe(1)
       expect(addEvents[0].textContent).toBe('a {color: red}')
-      const styleElements = manager.getStyleElements()
+      const styleElements = styleManager.getStyleElements()
       expect(styleElements.length).toBe(1)
       expect(styleElements[0].textContent).toBe('a {color: red}')
       disposable.dispose()
       expect(removeEvents.length).toBe(1)
       expect(removeEvents[0].textContent).toBe('a {color: red}')
-      expect(manager.getStyleElements().length).toBe(0)
+      expect(styleManager.getStyleElements().length).toBe(0)
+    })
+
+    describe('atom-text-editor shadow DOM selectors upgrades', () => {
+      beforeEach(() => {
+        // attach styles element to the DOM to parse CSS rules
+        styleManager.onDidAddStyleElement((styleElement) => { jasmine.attachToDOM(styleElement) })
+      })
+
+      it('removes the ::shadow pseudo-element from atom-text-editor selectors', () => {
+          styleManager.addStyleSheet(`
+            atom-text-editor::shadow .class-1, atom-text-editor::shadow .class-2 { color: red }
+            atom-text-editor::shadow > .class-3 { color: yellow }
+            atom-text-editor .class-4 { color: blue }
+            another-element::shadow .class-5 { color: white }
+          `)
+          expect(Array.from(styleManager.getStyleElements()[0].sheet.cssRules).map((r) => r.selectorText)).toEqual([
+            'atom-text-editor .class-1, atom-text-editor .class-2',
+            'atom-text-editor > .class-3',
+            'atom-text-editor .class-4',
+            'another-element::shadow .class-5'
+          ])
+        }
+      )
+
+      describe('when a selector targets the atom-text-editor shadow DOM', () => {
+        it('prepends "--syntax" to class selectors matching a grammar scope name and not already starting with "syntax--"', () => {
+          styleManager.addStyleSheet(`
+            .class-1 { color: red }
+            .source > .js, .source.coffee { color: green }
+            .syntax--source { color: gray }
+            #id-1 { color: blue }
+          `, {context: 'atom-text-editor'})
+          expect(Array.from(styleManager.getStyleElements()[0].sheet.cssRules).map((r) => r.selectorText)).toEqual([
+            '.class-1',
+            '.syntax--source > .syntax--js, .syntax--source.syntax--coffee',
+            '.syntax--source',
+            '#id-1'
+          ])
+
+          styleManager.addStyleSheet(`
+            .source > .js, .source.coffee { color: green }
+            atom-text-editor::shadow .source > .js { color: yellow }
+            atom-text-editor .source > .js { color: red }
+          `)
+          expect(Array.from(styleManager.getStyleElements()[1].sheet.cssRules).map((r) => r.selectorText)).toEqual([
+            '.source > .js, .source.coffee',
+            'atom-text-editor .syntax--source > .syntax--js',
+            'atom-text-editor .source > .js'
+          ])
+        })
+      })
+
+      it('replaces ":host" with "atom-text-editor" only when the context of a stylesheet is "atom-text-editor"', () => {
+        styleManager.addStyleSheet(':host .class-1, :host .class-2 { color: red; }')
+        expect(Array.from(styleManager.getStyleElements()[0].sheet.cssRules).map((r) => r.selectorText)).toEqual([
+          ':host .class-1, :host .class-2'
+        ])
+        global.debug = true
+        styleManager.addStyleSheet(':host .class-1, :host .class-2 { color: red; }', {context: 'atom-text-editor'})
+        global.debug = false
+        expect(Array.from(styleManager.getStyleElements()[1].sheet.cssRules).map((r) => r.selectorText)).toEqual([
+          'atom-text-editor .class-1, atom-text-editor .class-2'
+        ])
+      })
+
+      it('does not throw exceptions on rules with no selectors', () => {
+        styleManager.addStyleSheet('@media screen {font-size: 10px}', {context: 'atom-text-editor'})
+      })
     })
 
     describe('when a sourcePath parameter is specified', () => {
       it('ensures a maximum of one style element for the given source path, updating a previous if it exists', () => {
-        const disposable1 = manager.addStyleSheet('a {color: red}', {sourcePath: '/foo/bar'})
+        const disposable1 = styleManager.addStyleSheet('a {color: red}', {sourcePath: '/foo/bar'})
         expect(addEvents.length).toBe(1)
         expect(addEvents[0].getAttribute('source-path')).toBe('/foo/bar')
 
-        const disposable2 = manager.addStyleSheet('a {color: blue}', {sourcePath: '/foo/bar'})
+        const disposable2 = styleManager.addStyleSheet('a {color: blue}', {sourcePath: '/foo/bar'})
         expect(addEvents.length).toBe(1)
         expect(updateEvents.length).toBe(1)
         expect(updateEvents[0].getAttribute('source-path')).toBe('/foo/bar')
@@ -41,7 +109,7 @@ describe('StyleManager', () => {
         disposable2.dispose()
 
         addEvents = []
-        manager.addStyleSheet('a {color: yellow}', {sourcePath: '/foo/bar'})
+        styleManager.addStyleSheet('a {color: yellow}', {sourcePath: '/foo/bar'})
         expect(addEvents.length).toBe(1)
         expect(addEvents[0].getAttribute('source-path')).toBe('/foo/bar')
         expect(addEvents[0].textContent).toBe('a {color: yellow}')
@@ -50,11 +118,11 @@ describe('StyleManager', () => {
 
     describe('when a priority parameter is specified', () => {
       it('inserts the style sheet based on the priority', () => {
-        manager.addStyleSheet('a {color: red}', {priority: 1})
-        manager.addStyleSheet('a {color: blue}', {priority: 0})
-        manager.addStyleSheet('a {color: green}', {priority: 2})
-        manager.addStyleSheet('a {color: yellow}', {priority: 1})
-        expect(manager.getStyleElements().map((elt) => elt.textContent)).toEqual([
+        styleManager.addStyleSheet('a {color: red}', {priority: 1})
+        styleManager.addStyleSheet('a {color: blue}', {priority: 0})
+        styleManager.addStyleSheet('a {color: green}', {priority: 2})
+        styleManager.addStyleSheet('a {color: yellow}', {priority: 1})
+        expect(styleManager.getStyleElements().map((elt) => elt.textContent)).toEqual([
           'a {color: blue}',
           'a {color: red}',
           'a {color: yellow}',
diff --git a/spec/styles-element-spec.coffee b/spec/styles-element-spec.coffee
index b29979524..0889b0d43 100644
--- a/spec/styles-element-spec.coffee
+++ b/spec/styles-element-spec.coffee
@@ -77,71 +77,3 @@ describe "StylesElement", ->
     expect(element.children.length).toBe 2
     expect(element.children[0].textContent).toBe "a {color: red;}"
     expect(element.children[1].textContent).toBe "a {color: blue;}"
-
-  describe "atom-text-editor shadow DOM selector upgrades", ->
-    beforeEach ->
-      spyOn(console, 'warn')
-
-    it "removes the ::shadow pseudo-element from atom-text-editor selectors", ->
-      atom.styles.addStyleSheet("""
-      atom-text-editor::shadow .class-1, atom-text-editor::shadow .class-2 { color: red; }
-      atom-text-editor::shadow > .class-3 { color: yellow; }
-      atom-text-editor .class-4 { color: blue; }
-      another-element::shadow .class-5 { color: white; }
-      """)
-      expect(Array.from(element.lastChild.sheet.cssRules).map((r) -> r.selectorText)).toEqual([
-        'atom-text-editor .class-1, atom-text-editor .class-2',
-        'atom-text-editor > .class-3',
-        'atom-text-editor .class-4',
-        'another-element::shadow .class-5'
-      ])
-      expect(console.warn).toHaveBeenCalled()
-
-    describe "when the context of a style sheet is 'atom-text-editor'", ->
-      it "prepends `--syntax` to selectors not contained in atom-text-editor or matching a spatial decoration", ->
-        atom.styles.addStyleSheet("""
-        .class-1 { color: red; }
-        .class-2 > .class-3, .class-4.class-5 { color: green; }
-        .class-6 atom-text-editor .class-7 { color: yellow; }
-        atom-text-editor .class-8, .class-9 { color: blue; }
-        atom-text-editor .indent-guide, atom-text-editor .leading-whitespace { background: white; }
-        .syntax--class-10 { color: gray; }
-        :host .class-11 { color: purple; }
-        #id-1 { color: gray; }
-        """, {context: 'atom-text-editor'})
-        expect(Array.from(element.lastChild.sheet.cssRules).map((r) -> r.selectorText)).toEqual([
-          '.syntax--class-1',
-          '.syntax--class-2 > .syntax--class-3, .syntax--class-4.syntax--class-5',
-          '.class-6 atom-text-editor .class-7',
-          'atom-text-editor .class-8, .syntax--class-9',
-          'atom-text-editor .syntax--indent-guide, atom-text-editor .syntax--leading-whitespace',
-          '.syntax--class-10',
-          'atom-text-editor .class-11',
-          '#id-1'
-        ])
-        expect(console.warn).toHaveBeenCalled()
-
-    describe "when the context of a style sheet is not 'atom-text-editor'", ->
-      it "never prepends class names with `--syntax`", ->
-        atom.styles.addStyleSheet("""
-        .class-1 { color: red; }
-        .class-2 > .class-3, .class-4.class-5 { color: green; }
-        .class-6 atom-text-editor .class-7 { color: yellow; }
-        atom-text-editor .class-8, .class-9 { color: blue; }
-        atom-text-editor .indent-guide, atom-text-editor .leading-whitespace { background: white; }
-        #id-1 { color: gray; }
-        """)
-        expect(Array.from(element.lastChild.sheet.cssRules).map((r) -> r.selectorText)).toEqual([
-          '.class-1'
-          '.class-2 > .class-3, .class-4.class-5'
-          '.class-6 atom-text-editor .class-7'
-          'atom-text-editor .class-8, .class-9'
-          'atom-text-editor .indent-guide, atom-text-editor .leading-whitespace'
-          '#id-1'
-        ])
-        expect(console.warn).not.toHaveBeenCalled()
-
-    it "does not throw exceptions on rules with no selectors", ->
-      atom.styles.addStyleSheet """
-        @media screen {font-size: 10px;}
-      """, context: 'atom-text-editor'
diff --git a/src/deprecated-syntax-selectors.js b/src/deprecated-syntax-selectors.js
index 8b85956ca..8b798bd4c 100644
--- a/src/deprecated-syntax-selectors.js
+++ b/src/deprecated-syntax-selectors.js
@@ -863,7 +863,7 @@ module.exports = new Set([
   'trace', 'trace-argument', 'trace-object', 'traceback', 'tracing',
   'track_processing', 'trader', 'tradersk', 'trail', 'trailing',
   'trailing-array-separator', 'trailing-dictionary-separator', 'trailing-match',
-  'trailing-whitespace', 'trait', 'traits', 'traits-keyword', 'transaction',
+  'trait', 'traits', 'traits-keyword', 'transaction',
   'transcendental', 'transcludeblock', 'transcludeinline', 'transclusion',
   'transform', 'transformation', 'transient', 'transition',
   'transitionable-property-value', 'translation', 'transmission-filter',
diff --git a/src/lines-tile-component.coffee b/src/lines-tile-component.coffee
index c7e3bf367..c1cb2ba64 100644
--- a/src/lines-tile-component.coffee
+++ b/src/lines-tile-component.coffee
@@ -202,8 +202,8 @@ class LinesTileComponent
       if @presenter.isCloseTagCode(tagCode)
         openScopeNode = openScopeNode.parentElement
       else if @presenter.isOpenTagCode(tagCode)
-        scopes = @presenter.tagForCode(tagCode).replace(/\s+/g, '.').split('.').map((scope) -> "syntax--#{scope}")
-        newScopeNode = @domElementPool.buildElement("span", scopes.join(' '))
+        scope = @presenter.tagForCode(tagCode)
+        newScopeNode = @domElementPool.buildElement("span", scope.replace(/\.+/g, ' '))
         openScopeNode.appendChild(newScopeNode)
         openScopeNode = newScopeNode
       else
@@ -219,7 +219,7 @@ class LinesTileComponent
 
     if lineText.endsWith(@presenter.displayLayer.foldCharacter)
       # Insert a zero-width non-breaking whitespace, so that
-      # LinesYardstick can take the syntax--fold-marker::after pseudo-element
+      # LinesYardstick can take the fold-marker::after pseudo-element
       # into account during measurements when such marker is the last
       # character on the line.
       textNode = @domElementPool.buildText(ZERO_WIDTH_NBSP)
diff --git a/src/style-manager.js b/src/style-manager.js
index ae06f9060..a294d1d15 100644
--- a/src/style-manager.js
+++ b/src/style-manager.js
@@ -1,7 +1,10 @@
+const {Emitter, Disposable} = require('event-kit')
 const fs = require('fs-plus')
 const path = require('path')
-const {Emitter, Disposable} = require('event-kit')
+const postcss = require('postcss')
+const selectorParser = require('postcss-selector-parser')
 const StylesElement = require('./styles-element')
+const DEPRECATED_SYNTAX_SELECTORS = require('./deprecated-syntax-selectors')
 
 // Extended: A singleton instance of this class available via `atom.styles`,
 // which you can use to globally query and observe the set of active style
@@ -122,7 +125,7 @@ module.exports = class StyleManager {
       }
     }
 
-    styleElement.textContent = source
+    styleElement.textContent = transformDeprecatedShadowDOMSelectors(source, params.context)
     if (updated) {
       this.emitter.emit('did-update-style-element', styleElement)
     } else {
@@ -205,3 +208,37 @@ module.exports = class StyleManager {
     }
   }
 }
+
+function transformDeprecatedShadowDOMSelectors (css, context) {
+  const root = postcss.parse(css)
+  root.walkRules((rule) => {
+    rule.selector = selectorParser((selectors) => {
+      selectors.each((selector) => {
+        const firstNode = selector.nodes[0]
+        if (context === 'atom-text-editor' && firstNode.type === 'pseudo' && firstNode.value === ':host') {
+          const atomTextEditorElementNode = selectorParser.tag({value: 'atom-text-editor'})
+          firstNode.replaceWith(atomTextEditorElementNode)
+        }
+
+        let targetsAtomTextEditorShadow = context === 'atom-text-editor'
+        let previousNode
+        selector.each((node) => {
+          if (targetsAtomTextEditorShadow && node.type === 'class') {
+            if (DEPRECATED_SYNTAX_SELECTORS.has(node.value) && !node.value.startsWith('syntax--')) {
+              node.value = `syntax--${node.value}`
+            }
+          } else if (previousNode) {
+            const currentNodeIsShadowPseudoClass = node.type === 'pseudo' && node.value === '::shadow'
+            const previousNodeIsAtomTextEditor = previousNode.type === 'tag' && previousNode.value === 'atom-text-editor'
+            if (previousNodeIsAtomTextEditor && currentNodeIsShadowPseudoClass) {
+              selector.removeChild(node)
+              targetsAtomTextEditorShadow = true
+            }
+          }
+          previousNode = node
+        })
+      })
+    }).process(rule.selector).result
+  })
+  return root.toString()
+}
diff --git a/src/styles-element.coffee b/src/styles-element.coffee
index e82775c92..2c53300c2 100644
--- a/src/styles-element.coffee
+++ b/src/styles-element.coffee
@@ -1,9 +1,4 @@
 {Emitter, CompositeDisposable} = require 'event-kit'
-selectorProcessor = require 'postcss-selector-parser'
-SPATIAL_DECORATIONS = new Set([
-  'invisible-character', 'hard-tab', 'leading-whitespace',
-  'trailing-whitespace', 'eol', 'indent-guide', 'fold-marker'
-])
 
 class StylesElement extends HTMLElement
   subscriptions: null
@@ -24,9 +19,6 @@ class StylesElement extends HTMLElement
     @styleElementClonesByOriginalElement = new WeakMap
 
   attachedCallback: ->
-    for styleElement in @children
-      @upgradeDeprecatedSelectors(styleElement)
-
     @context = @getAttribute('context') ? undefined
 
   detachedCallback: ->
@@ -68,7 +60,6 @@ class StylesElement extends HTMLElement
           break
 
     @insertBefore(styleElementClone, insertBefore)
-    @upgradeDeprecatedSelectors(styleElementClone)
     @emitter.emit 'did-add-style-element', styleElementClone
 
   styleElementRemoved: (styleElement) ->
@@ -88,52 +79,4 @@ class StylesElement extends HTMLElement
   styleElementMatchesContext: (styleElement) ->
     not @context? or styleElement.context is @context
 
-  upgradeDeprecatedSelectors: (styleElement) ->
-    return unless styleElement.sheet?
-
-    transformDeprecatedShadowSelectors = (selectors) ->
-      selectors.each (selector) ->
-        isSyntaxSelector = not selector.some((node) ->
-          (node.type is 'tag' and node.value is 'atom-text-editor') or
-          (node.type is 'class' and node.value is 'region') or
-          (node.type is 'class' and node.value is 'wrap-guide') or
-          (node.type is 'class' and /spell-check/.test(node.value))
-        )
-        previousNode = null
-        selector.each (node) ->
-          isShadowPseudoClass = node.type is 'pseudo' and node.value is '::shadow'
-          isHostPseudoClass = node.type is 'pseudo' and node.value is ':host'
-          if isHostPseudoClass and not previousNode?
-            newNode = selectorProcessor.tag({value: 'atom-text-editor'})
-            node.replaceWith(newNode)
-            previousNode = newNode
-          else if isShadowPseudoClass and previousNode?.type is 'tag' and previousNode?.value is 'atom-text-editor'
-            selector.removeChild(node)
-          else
-            if styleElement.context is 'atom-text-editor' and node.type is 'class'
-              if (isSyntaxSelector and not node.value.startsWith('syntax--')) or SPATIAL_DECORATIONS.has(node.value)
-                node.value = 'syntax--' + node.value
-            previousNode = node
-
-    upgradedSelectors = []
-    for rule in styleElement.sheet.cssRules when rule.selectorText?
-      inputSelector = rule.selectorText
-      outputSelector = rule.selectorText
-      outputSelector = selectorProcessor(transformDeprecatedShadowSelectors).process(outputSelector).result
-      if inputSelector isnt outputSelector
-        rule.selectorText = outputSelector
-        upgradedSelectors.push({inputSelector, outputSelector})
-
-    if upgradedSelectors.length > 0
-      upgradedSelectorsText = upgradedSelectors.map(({inputSelector, outputSelector}) -> "`#{inputSelector}` => `#{outputSelector}`").join('\n')
-      console.warn("""
-      Shadow DOM for `atom-text-editor` elements has been removed. This means
-      should stop using :host and ::shadow pseudo-selectors, and prepend all
-      your syntax selectors with `syntax--`. To prevent breakage with existing
-      stylesheets, we have automatically upgraded the following selectors in
-      `#{styleElement.sourcePath}`:
-
-      #{upgradedSelectorsText}
-      """)
-
 module.exports = StylesElement = document.registerElement 'atom-styles', prototype: StylesElement.prototype
diff --git a/static/text-editor-light.less b/static/text-editor-light.less
index 1f16ec71e..193749d51 100644
--- a/static/text-editor-light.less
+++ b/static/text-editor-light.less
@@ -122,12 +122,12 @@ atom-text-editor {
   .line {
     white-space: pre;
 
-    &.cursor-line .syntax--fold-marker::after {
+    &.cursor-line .fold-marker::after {
       opacity: 1;
     }
   }
 
-  .syntax--fold-marker {
+  .fold-marker {
     cursor: default;
 
     &::after {
@@ -143,12 +143,12 @@ atom-text-editor {
     color: @text-color-subtle;
   }
 
-  .syntax--invisible-character {
+  .invisible-character {
     font-weight: normal !important;
     font-style: normal !important;
   }
 
-  .syntax--indent-guide {
+  .indent-guide {
     display: inline-block;
     box-shadow: inset 1px 0;
   }