Implement style computing (#1399)

Ref https://github.com/svg/svgo/issues/777

Currently a lot of optimisations are attributes specific and may be
broken because of inline or shared styles.

In this diff I'm trying to solve the problem with getComputedStyle
analog.

`computeStyle` collects attributes, shared css rules, inline styles
and inherited styles and checks whether they can be statically optimised
or left as deoptimisation.

lib/style.js

@@ -0,0 +1,184 @@
+'use strict';
+
+const stable = require('stable');
+const csstree = require('css-tree');
+const specificity = require('csso/lib/restructure/prepare/specificity');
+const { selectAll, is } = require('css-select');
+const svgoCssSelectAdapter = require('./svgo/css-select-adapter.js');
+const { compareSpecificity } = require('./css-tools.js');
+const {
+  attrsGroups,
+  inheritableAttrs,
+  presentationNonInheritableGroupAttrs,
+} = require('../plugins/_collections.js');
+
+const cssSelectOptions = {
+  xmlMode: true,
+  adapter: svgoCssSelectAdapter,
+};
+
+const parseRule = (ruleNode, dynamic) => {
+  let selectors;
+  let selectorsSpecificity;
+  const declarations = [];
+  csstree.walk(ruleNode, (cssNode) => {
+    if (cssNode.type === 'SelectorList') {
+      // compute specificity from original node to consider pseudo classes
+      selectorsSpecificity = specificity(cssNode);
+      const newSelectorsNode = csstree.clone(cssNode);
+      csstree.walk(newSelectorsNode, (pseudoClassNode, item, list) => {
+        if (pseudoClassNode.type === 'PseudoClassSelector') {
+          dynamic = true;
+          list.remove(item);
+        }
+      });
+      selectors = csstree.generate(newSelectorsNode);
+      return csstree.walk.skip;
+    }
+    if (cssNode.type === 'Declaration') {
+      declarations.push({
+        name: cssNode.property,
+        value: csstree.generate(cssNode.value),
+        important: cssNode.important,
+      });
+      return csstree.walk.skip;
+    }
+  });
+  return {
+    dynamic,
+    selectors,
+    specificity: selectorsSpecificity,
+    declarations,
+  };
+};
+
+const parseStylesheet = (css, dynamic) => {
+  const rules = [];
+  const ast = csstree.parse(css);
+  csstree.walk(ast, (cssNode) => {
+    if (cssNode.type === 'Rule') {
+      rules.push(parseRule(cssNode, dynamic || false));
+      return csstree.walk.skip;
+    }
+    if (cssNode.type === 'Atrule') {
+      csstree.walk(cssNode, (ruleNode) => {
+        if (ruleNode.type === 'Rule') {
+          rules.push(parseRule(ruleNode, dynamic || true));
+          return csstree.walk.skip;
+        }
+      });
+      return csstree.walk.skip;
+    }
+  });
+  return rules;
+};
+
+const computeOwnStyle = (node, stylesheet) => {
+  const computedStyle = {};
+  const importantStyles = new Map();
+
+  // collect attributes
+  if (node.attrs) {
+    for (const { name, value } of Object.values(node.attrs)) {
+      if (attrsGroups.presentation.includes(name)) {
+        computedStyle[name] = { type: 'static', inherited: false, value };
+        importantStyles.set(name, false);
+      }
+    }
+  }
+
+  // collect matching rules
+  for (const { selectors, declarations, dynamic } of stylesheet) {
+    if (is(node, selectors, cssSelectOptions)) {
+      for (const { name, value, important } of declarations) {
+        const computed = computedStyle[name];
+        if (computed && computed.type === 'dynamic') {
+          continue;
+        }
+        if (dynamic) {
+          computedStyle[name] = { type: 'dynamic', inherited: false };
+          continue;
+        }
+        if (
+          computed == null ||
+          important === true ||
+          importantStyles.get(name) === false
+        ) {
+          computedStyle[name] = { type: 'static', inherited: false, value };
+          importantStyles.set(name, important);
+        }
+      }
+    }
+  }
+
+  // collect inline styles
+  for (const [name, { value, priority }] of node.style.properties) {
+    const computed = computedStyle[name];
+    const important = priority === 'important';
+    if (computed && computed.type === 'dynamic') {
+      continue;
+    }
+    if (
+      computed == null ||
+      important === true ||
+      importantStyles.get(name) === false
+    ) {
+      computedStyle[name] = { type: 'static', inherited: false, value };
+      importantStyles.set(name, important);
+    }
+  }
+
+  return computedStyle;
+};
+
+const computeStyle = (node) => {
+  // find root
+  let root = node;
+  while (root.parentNode) {
+    root = root.parentNode;
+  }
+  // find all styles
+  const styleNodes = selectAll('style', root, cssSelectOptions);
+  // parse all styles
+  const stylesheet = [];
+  for (const styleNode of styleNodes) {
+    const dynamic =
+      styleNode.hasAttr('media') && styleNode.attr('media').value !== 'all';
+    if (
+      styleNode.hasAttr('type') === false ||
+      styleNode.attr('type').value === '' ||
+      styleNode.attr('type').value === 'text/css'
+    ) {
+      const children = styleNode.content || [];
+      for (const child of children) {
+        const css = child.text || child.cdata;
+        stylesheet.push(...parseStylesheet(css, dynamic));
+      }
+    }
+  }
+  // sort by selectors specificity
+  stable.inplace(stylesheet, (a, b) =>
+    compareSpecificity(a.specificity, b.specificity)
+  );
+
+  // collect inherited styles
+  const computedStyles = computeOwnStyle(node, stylesheet);
+  let parent = node;
+  while (parent.parentNode && parent.parentNode.elem !== '#document') {
+    const inheritedStyles = computeOwnStyle(parent.parentNode, stylesheet);
+    for (const [name, computed] of Object.entries(inheritedStyles)) {
+      if (
+        computedStyles[name] == null &&
+        // ignore not inheritable styles
+        inheritableAttrs.includes(name) === true &&
+        presentationNonInheritableGroupAttrs.includes(name) === false
+      ) {
+        computedStyles[name] = { ...computed, inherited: true };
+      }
+    }
+    parent = parent.parentNode;
+  }
+
+  return computedStyles;
+};
+exports.computeStyle = computeStyle;

lib/style.test.js

@@ -0,0 +1,215 @@
+'use strict';
+
+const { expect } = require('chai');
+const { computeStyle } = require('./style.js');
+const svg2js = require('./svgo/svg2js.js');
+
+describe('computeStyle', () => {
+  it('collects styles', () => {
+    const root = svg2js(`
+      <svg>
+        <rect id="class" class="a" />
+        <rect id="two-classes" class="b a" />
+        <rect id="attribute" fill="purple" />
+        <rect id="inline-style" style="fill: grey;" />
+        <g fill="yellow">
+          <rect id="inheritance" />
+          <g style="fill: blue;">
+            <g>
+              <rect id="nested-inheritance" />
+            </g>
+          </g>
+        </g>
+        <style>
+          .a { fill: red; }
+        </style>
+        <style>
+          <![CDATA[
+            .b { fill: green; stroke: black; }
+          ]]>
+        </style>
+      </svg>
+    `);
+    expect(computeStyle(root.querySelector('#class'))).to.deep.equal({
+      fill: { type: 'static', inherited: false, value: 'red' },
+    });
+    expect(computeStyle(root.querySelector('#two-classes'))).to.deep.equal({
+      fill: { type: 'static', inherited: false, value: 'green' },
+      stroke: { type: 'static', inherited: false, value: 'black' },
+    });
+    expect(computeStyle(root.querySelector('#attribute'))).to.deep.equal({
+      fill: { type: 'static', inherited: false, value: 'purple' },
+    });
+    expect(computeStyle(root.querySelector('#inline-style'))).to.deep.equal({
+      fill: { type: 'static', inherited: false, value: 'grey' },
+    });
+    expect(computeStyle(root.querySelector('#inheritance'))).to.deep.equal({
+      fill: { type: 'static', inherited: true, value: 'yellow' },
+    });
+    expect(
+      computeStyle(root.querySelector('#nested-inheritance'))
+    ).to.deep.equal({
+      fill: { type: 'static', inherited: true, value: 'blue' },
+    });
+  });
+
+  it('prioritizes different kinds of styles', () => {
+    const root = svg2js(`
+      <svg>
+        <style>
+          g > .a { fill: red; }
+          .a { fill: green; }
+          .b { fill: blue; }
+        </style>
+        <g fill="yellow">
+          <rect id="complex-selector" class="a" />
+          <rect id="attribute-over-inheritance" fill="orange" />
+          <rect id="style-rule-over-attribute" class="b" fill="grey" />
+          <rect id="inline-style-over-style-rule" style="fill: purple;" class="b" />
+        </g>
+      </svg>
+    `);
+    expect(computeStyle(root.querySelector('#complex-selector'))).to.deep.equal(
+      {
+        fill: { type: 'static', inherited: false, value: 'red' },
+      }
+    );
+    expect(
+      computeStyle(root.querySelector('#attribute-over-inheritance'))
+    ).to.deep.equal({
+      fill: { type: 'static', inherited: false, value: 'orange' },
+    });
+    expect(
+      computeStyle(root.querySelector('#style-rule-over-attribute'))
+    ).to.deep.equal({
+      fill: { type: 'static', inherited: false, value: 'blue' },
+    });
+    expect(
+      computeStyle(root.querySelector('#inline-style-over-style-rule'))
+    ).to.deep.equal({
+      fill: { type: 'static', inherited: false, value: 'purple' },
+    });
+  });
+
+  it('prioritizes important styles', () => {
+    const root = svg2js(`
+      <svg>
+        <style>
+          g > .a { fill: red; }
+          .b { fill: green !important; }
+        </style>
+        <rect id="complex-selector" class="a b" />
+        <rect id="style-rule-over-inline-style" style="fill: orange;" class="b" />
+        <rect id="inline-style-over-style-rule" style="fill: purple !important;" class="b" />
+      </svg>
+    `);
+    expect(computeStyle(root.querySelector('#complex-selector'))).to.deep.equal(
+      {
+        fill: { type: 'static', inherited: false, value: 'green' },
+      }
+    );
+    expect(
+      computeStyle(root.querySelector('#style-rule-over-inline-style'))
+    ).to.deep.equal({
+      fill: { type: 'static', inherited: false, value: 'green' },
+    });
+    expect(
+      computeStyle(root.querySelector('#inline-style-over-style-rule'))
+    ).to.deep.equal({
+      fill: { type: 'static', inherited: false, value: 'purple' },
+    });
+  });
+
+  it('treats at-rules and pseudo-classes as dynamic styles', () => {
+    const root = svg2js(`
+      <svg>
+        <style>
+          @media screen {
+            .a { fill: red; }
+          }
+          .b:hover { fill: green; }
+          .c { fill: blue; }
+          .d { fill: purple; }
+        </style>
+        <rect id="media-query" class="a d" style="fill: orange;" />
+        <rect id="hover" class="b" style="fill: yellow;" />
+        <g class="a">
+          <rect id="inherited" />
+          <rect id="inherited-overriden" class="c" />
+        </g>
+        <rect id="static" class="c" style="fill: black" />
+      </svg>
+    `);
+    expect(computeStyle(root.querySelector('#media-query'))).to.deep.equal({
+      fill: { type: 'dynamic', inherited: false },
+    });
+    expect(computeStyle(root.querySelector('#hover'))).to.deep.equal({
+      fill: { type: 'dynamic', inherited: false },
+    });
+    expect(computeStyle(root.querySelector('#inherited'))).to.deep.equal({
+      fill: { type: 'dynamic', inherited: true },
+    });
+    expect(
+      computeStyle(root.querySelector('#inherited-overriden'))
+    ).to.deep.equal({
+      fill: { type: 'static', inherited: false, value: 'blue' },
+    });
+    expect(computeStyle(root.querySelector('#static'))).to.deep.equal({
+      fill: { type: 'static', inherited: false, value: 'black' },
+    });
+  });
+
+  it('considers <style> media attribute', () => {
+    const root = svg2js(`
+      <svg>
+        <style media="print">
+          @media screen {
+            .a { fill: red; }
+          }
+          .b { fill: green; }
+        </style>
+        <style media="all">
+          .c { fill: blue; }
+        </style>
+        <rect id="media-query" class="a" />
+        <rect id="kinda-static" class="b" />
+        <rect id="static" class="c" />
+      </svg>
+    `);
+    expect(computeStyle(root.querySelector('#media-query'))).to.deep.equal({
+      fill: { type: 'dynamic', inherited: false },
+    });
+    expect(computeStyle(root.querySelector('#kinda-static'))).to.deep.equal({
+      fill: { type: 'dynamic', inherited: false },
+    });
+    expect(computeStyle(root.querySelector('#static'))).to.deep.equal({
+      fill: { type: 'static', inherited: false, value: 'blue' },
+    });
+  });
+
+  it('ignores <style> with invalid type', () => {
+    const root = svg2js(`
+      <svg>
+        <style type="text/css">
+          .a { fill: red; }
+        </style>
+        <style type="">
+          .b { fill: green; }
+        </style>
+        <style type="text/invalid">
+          .c { fill: blue; }
+        </style>
+        <rect id="valid-type" class="a" />
+        <rect id="empty-type" class="b" />
+        <rect id="invalid-type" class="c" />
+      </svg>
+    `);
+    expect(computeStyle(root.querySelector('#valid-type'))).to.deep.equal({
+      fill: { type: 'static', inherited: false, value: 'red' },
+    });
+    expect(computeStyle(root.querySelector('#empty-type'))).to.deep.equal({
+      fill: { type: 'static', inherited: false, value: 'green' },
+    });
+    expect(computeStyle(root.querySelector('#invalid-type'))).to.deep.equal({});
+  });
+});

plugins/removeHiddenElems.js

@@ -1,10 +1,13 @@
 'use strict';
 
+const { computeStyle } = require('../lib/style.js');
+
 exports.type = 'perItem';
 
 exports.active = true;
 
-exports.description = 'removes hidden elements (zero sized, with absent attributes)';
+exports.description =
+  'removes hidden elements (zero sized, with absent attributes)';
 
 exports.params = {
   isHidden: true,
@@ -21,7 +24,7 @@ exports.params = {
   imageHeight0: true,
   pathEmptyD: true,
   polylineEmptyPoints: true,
-    polygonEmptyPoints: true
+  polygonEmptyPoints: true,
 };
 
 var regValidPath = /M\s*(?:[-+]?(?:\d*\.\d+|\d+(?:\.|(?!\.)))([eE][-+]?\d+)?(?!\d)\s*,?\s*){2}\D*\d/i;
@@ -46,16 +49,20 @@ var regValidPath = /M\s*(?:[-+]?(?:\d*\.\d+|\d+(?:\.|(?!\.)))([eE][-+]?\d+)?(?!\
  * @author Kir Belevich
  */
 exports.fn = function (item, params) {
-
   if (item.elem) {
     // Removes hidden elements
     // https://www.w3schools.com/cssref/pr_class_visibility.asp
+    const computedStyle = computeStyle(item);
     if (
       params.isHidden &&
-            item.hasAttr('visibility', 'hidden') &&
+      computedStyle.visibility &&
+      computedStyle.visibility.type === 'static' &&
+      computedStyle.visibility.value === 'hidden' &&
       // keep if any descendant enables visibility
       item.querySelector('[visibility=visible]') == null
-        ) return false;
+    ) {
+      return false;
+    }
 
     // display="none"
     //
@@ -64,18 +71,26 @@ exports.fn = function (item, params) {
     // and its children shall not be rendered directly"
     if (
       params.displayNone &&
-            item.hasAttr('display', 'none')
-        ) return false;
+      computedStyle.display &&
+      computedStyle.display.type === 'static' &&
+      computedStyle.display.value === 'none'
+    ) {
+      return false;
+    }
 
     // opacity="0"
     //
     // https://www.w3.org/TR/SVG11/masking.html#ObjectAndGroupOpacityProperties
     if (
       params.opacity0 &&
-            item.hasAttr('opacity', '0') &&
+      computedStyle.opacity &&
+      computedStyle.opacity.type === 'static' &&
+      computedStyle.opacity.value === '0' &&
       // transparent element inside clipPath still affect clipped elements
       item.closestElem('clipPath') == null
-        ) return false;
+    ) {
+      return false;
+    }
 
     // Circles with zero radius
     //
@@ -88,7 +103,9 @@ exports.fn = function (item, params) {
       item.isElem('circle') &&
       item.isEmpty() &&
       item.hasAttr('r', '0')
-        ) return false;
+    ) {
+      return false;
+    }
 
     // Ellipse with zero x-axis radius
     //
@@ -101,7 +118,9 @@ exports.fn = function (item, params) {
       item.isElem('ellipse') &&
       item.isEmpty() &&
       item.hasAttr('rx', '0')
-        ) return false;
+    ) {
+      return false;
+    }
 
     // Ellipse with zero y-axis radius
     //
@@ -114,7 +133,9 @@ exports.fn = function (item, params) {
       item.isElem('ellipse') &&
       item.isEmpty() &&
       item.hasAttr('ry', '0')
-        ) return false;
+    ) {
+      return false;
+    }
 
     // Rectangle with zero width
     //
@@ -127,7 +148,9 @@ exports.fn = function (item, params) {
       item.isElem('rect') &&
       item.isEmpty() &&
       item.hasAttr('width', '0')
-        ) return false;
+    ) {
+      return false;
+    }
 
     // Rectangle with zero height
     //
@@ -141,7 +164,9 @@ exports.fn = function (item, params) {
       item.isElem('rect') &&
       item.isEmpty() &&
       item.hasAttr('height', '0')
-        ) return false;
+    ) {
+      return false;
+    }
 
     // Pattern with zero width
     //
@@ -153,7 +178,9 @@ exports.fn = function (item, params) {
       params.patternWidth0 &&
       item.isElem('pattern') &&
       item.hasAttr('width', '0')
-        ) return false;
+    ) {
+      return false;
+    }
 
     // Pattern with zero height
     //
@@ -165,7 +192,9 @@ exports.fn = function (item, params) {
       params.patternHeight0 &&
       item.isElem('pattern') &&
       item.hasAttr('height', '0')
-        ) return false;
+    ) {
+      return false;
+    }
 
     // Image with zero width
     //
@@ -177,7 +206,9 @@ exports.fn = function (item, params) {
       params.imageWidth0 &&
       item.isElem('image') &&
       item.hasAttr('width', '0')
-        ) return false;
+    ) {
+      return false;
+    }
 
     // Image with zero height
     //
@@ -189,7 +220,9 @@ exports.fn = function (item, params) {
       params.imageHeight0 &&
       item.isElem('image') &&
       item.hasAttr('height', '0')
-        ) return false;
+    ) {
+      return false;
+    }
 
     // Path with empty data
     //
@@ -200,7 +233,9 @@ exports.fn = function (item, params) {
       params.pathEmptyD &&
       item.isElem('path') &&
       (!item.hasAttr('d') || !regValidPath.test(item.attr('d').value))
-        ) return false;
+    ) {
+      return false;
+    }
 
     // Polyline with empty points
     //
@@ -211,7 +246,9 @@ exports.fn = function (item, params) {
       params.polylineEmptyPoints &&
       item.isElem('polyline') &&
       !item.hasAttr('points')
-        ) return false;
+    ) {
+      return false;
+    }
 
     // Polygon with empty points
     //
@@ -222,8 +259,8 @@ exports.fn = function (item, params) {
       params.polygonEmptyPoints &&
       item.isElem('polygon') &&
       !item.hasAttr('points')
-        ) return false;
-
+    ) {
+      return false;
+    }
   }
-
 };

test/plugins/removeHiddenElems.01.svg

@@ -1,11 +1,20 @@
 <svg xmlns="http://www.w3.org/2000/svg">
+    <style>
+      .a { display: block; }
+    </style>
     <g>
-        <path display="none" d="..."/>
+        <rect display="none" x="0" y="0" width="20" height="20" />
+        <rect display="none" class="a" x="0" y="0" width="20" height="20" />
     </g>
 </svg>
 
 @@@
 
 <svg xmlns="http://www.w3.org/2000/svg">
-    <g/>
+    <style>
+        .a { display: block; }
+    </style>
+    <g>
+        <rect display="none" class="a" x="0" y="0" width="20" height="20"/>
+    </g>
 </svg>

test/plugins/removeHiddenElems.02.svg

@@ -1,11 +1,20 @@
 <svg xmlns="http://www.w3.org/2000/svg">
+    <style>
+      .a { opacity: 0.5; }
+    </style>
     <g>
-        <path opacity="0" d="..."/>
+        <rect opacity="0" x="0" y="0" width="20" height="20" />
+        <rect opacity="0" class="a" x="0" y="0" width="20" height="20" />
     </g>
 </svg>
 
 @@@
 
 <svg xmlns="http://www.w3.org/2000/svg">
-    <g/>
+    <style>
+        .a { opacity: 0.5; }
+    </style>
+    <g>
+        <rect opacity="0" class="a" x="0" y="0" width="20" height="20"/>
+    </g>
 </svg>

test/plugins/removeHiddenElems.12.svg

@@ -1,4 +1,12 @@
+Keep invisible elements which have visibile ones inside
+and resolve styles
+
+===
+
 <svg width="480" height="360" xmlns="http://www.w3.org/2000/svg">
+  <style>
+    .a { visibility: visible; }
+  </style>
   <rect x="96" y="96" width="96" height="96" fill="lime" />
   <g visibility="hidden">
     <rect x="96" y="96" width="96" height="96" fill="red" />
@@ -7,14 +15,19 @@
   <g visibility="hidden">
     <rect x="196" y="196" width="96" height="96" fill="lime" visibility="visible" />
   </g>
+  <rect x="96" y="96" width="96" height="96" visibility="hidden" class="a" />
 </svg>
 
 @@@
 
 <svg width="480" height="360" xmlns="http://www.w3.org/2000/svg">
+    <style>
+        .a { visibility: visible; }
+    </style>
     <rect x="96" y="96" width="96" height="96" fill="lime"/>
     <rect x="196.5" y="196.5" width="95" height="95" fill="red"/>
     <g visibility="hidden">
         <rect x="196" y="196" width="96" height="96" fill="lime" visibility="visible"/>
     </g>
+    <rect x="96" y="96" width="96" height="96" visibility="hidden" class="a"/>
 </svg>