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Adding a new style property

Document history:

NOTE: This document is still missing a few pieces. I need to add information on adding to nsComputedDOMStyle.


When a new style property is needed there are many places in the code that need to be updated. This document outlines the procedure used to add a new property, in this case the property is a proprietary one called '-moz-force-broken-image-icons' and is used as a way for a stylesheet to force broken image icons to be displayed. This is all being done in the context of bug 58646.


Up front you have to decide some things about the new property:


  1. Is the property proprietary or specified by the CSS standard?
  2. Is the property inherited?
  3. What types of values can the property have?
  4. Does it logically fit with other existing properties?
  5. What is the impact to the layout of a page if that property changes?
  6. What do you want to name it?


  1. In our specific case, we want a property that is used internally, so it is a proprietary property.
  2. The property is to be used for images, which are leaf elements, so there is no need to inherit it.
  3. The property is used simply to force a broken image to be represented by an icon, so it only supports the values '0' and '1' as numerics. 
  4. It is hard to see how this property fits logically in with other properties, but if we stretch our imaginations we could say that it is a sort of UI property.
  5. If this property changes, the image frame has to be recreated. This is because the decision about whether to display the icon or not will impact the decision to replace the image frame with an inline text frame for the ALT text, so if the ALT text inline is already made, there is no image frame left around to reflow or otherwise modify.
  6. Finally, the name will be '-moz-force-broken-image-icons' - that should be pretty self-describing (by convention we start proprietary property names with '-moz-').


There are several places that need to be educated about a new style property. They are:

CSS Property Name / Constants / Hints

First, add the new name to the property list in nsCSSPropList.h  Insert the property in the list alphabetically, using the existing property names as a template. The format of the entry you will create is:

CSS_PROP(-moz-force-broken-image-icons, force_broken_image_icons, MozForceBrokenImageIcons, nsChangeHint_ReconstructFrame) // bug 58646

The first value is the formal property name, in other words the property name as it is seen by the CSS parser.
The second value is the name of the property as it will appear internally.
The third value is the name of the DOM property used to access your style.
The last value indicates what must change when the value of the property changes. It should be an nsChangeHint.

If you need to introduce new constants for the values of the property, they must be added to nsStyleConsts.h and to the appropriate keyword tables in nsCSSProps.cpp (note: this cookbook does not do this since the new property does not require any new keywords for the property values).

CSS Declaration

Changes will need to be made to the structs and classes defined in nsCSSDeclaration.h and nsCSSDeclaration.cpp

First, find the declaration of the struct that will hold the new property value (in the header file). For this example it is the struct nsCSSUserInterface . Modify the struct declaration to include a new data member for the new property, of the type CSSValue. Next, open the implementation file (the cpp) and modify the struct's constructors.

Next, the AppendValue method must be updated to support your new property. The CSSParser will call this to build up a declaration. Find the portion of that method that deals with the other properties in the struct that you are adding your property to (or create a new section if you are creating a new style struct). For this example we will find the 'UserInterface' section and add our new property there .
    // nsCSSUserInterface
    case eCSSProperty_user_input:
    case eCSSProperty_user_modify:
    case eCSSProperty_user_select:
    case eCSSProperty_key_equivalent:
    case eCSSProperty_user_focus:
    case eCSSProperty_resizer:
    case eCSSProperty_cursor:
    case eCSSProperty_force_broken_image_icons: {
      CSS_ENSURE(UserInterface) {
        switch (aProperty) {
          case eCSSProperty_user_input:       theUserInterface->mUserInput = aValue;      break;
          case eCSSProperty_user_modify:      theUserInterface->mUserModify = aValue;     break;
          case eCSSProperty_user_select:      theUserInterface->mUserSelect = aValue;     break;
          case eCSSProperty_key_equivalent: 
            CSS_ENSURE_DATA(theUserInterface->mKeyEquivalent, nsCSSValueList) {
              theUserInterface->mKeyEquivalent->mValue = aValue;
          case eCSSProperty_user_focus:       theUserInterface->mUserFocus = aValue;      break;
          case eCSSProperty_resizer:          theUserInterface->mResizer = aValue;        break;
          case eCSSProperty_cursor:
            CSS_ENSURE_DATA(theUserInterface->mCursor, nsCSSValueList) {
              theUserInterface->mCursor->mValue = aValue;
          case eCSSProperty_force_broken_image_icons: theUserInterface->mForceBrokenImageIcon = aValue; break;

          CSS_BOGUS_DEFAULT; // make compiler happy

The GetValue method must be similarly modified :
    // nsCSSUserInterface
    case eCSSProperty_user_input:
    case eCSSProperty_user_modify:
    case eCSSProperty_user_select:
    case eCSSProperty_key_equivalent:
    case eCSSProperty_user_focus:
    case eCSSProperty_resizer:
    case eCSSProperty_cursor:
    case eCSSProperty_force_broken_image_icons: {
      if (nullptr != theUserInterface) {
        switch (aProperty) {
          case eCSSProperty_user_input:       aValue = theUserInterface->mUserInput;       break;
          case eCSSProperty_user_modify:      aValue = theUserInterface->mUserModify;      break;
          case eCSSProperty_user_select:      aValue = theUserInterface->mUserSelect;      break;
          case eCSSProperty_key_equivalent:
            if (nullptr != theUserInterface->mKeyEquivalent) {
              aValue = theUserInterface->mKeyEquivalent->mValue;
          case eCSSProperty_user_focus:       aValue = theUserInterface->mUserFocus;       break;
          case eCSSProperty_resizer:          aValue = theUserInterface->mResizer;         break;
          case eCSSProperty_cursor:
            if (nullptr != theUserInterface->mCursor) {
              aValue = theUserInterface->mCursor->mValue;
          case eCSSProperty_force_broken_image_icons: aValue = theUserInterface->mForceBrokenImageIcons; break;

          CSS_BOGUS_DEFAULT; // make compiler happy
      else {

Finally modify the 'List' method to output the property value.
void nsCSSUserInterface::List(FILE* out, int32_t aIndent) const
  for (int32_t index = aIndent; --index >= 0; ) fputs("  ", out);

  nsAutoString buffer;

  mUserInput.AppendToString(buffer, eCSSProperty_user_input);
  mUserModify.AppendToString(buffer, eCSSProperty_user_modify);
  mUserSelect.AppendToString(buffer, eCSSProperty_user_select);
  nsCSSValueList*  keyEquiv = mKeyEquivalent;
  while (nullptr != keyEquiv) {
    keyEquiv->mValue.AppendToString(buffer, eCSSProperty_key_equivalent);
    keyEquiv= keyEquiv->mNext;
  mUserFocus.AppendToString(buffer, eCSSProperty_user_focus);
  mResizer.AppendToString(buffer, eCSSProperty_resizer);
  nsCSSValueList*  cursor = mCursor;
  while (nullptr != cursor) {
    cursor->mValue.AppendToString(buffer, eCSSProperty_cursor);
    cursor = cursor->mNext;


  fputs(NS_LossyConvertUTF16toASCII(buffer).get(), out);

CSS Parser

Next, the CSSParser must be educated about this new property so that it can read in the formal declarations and build up the internal declarations that will be used to build the rules. If you are adding a simple property that takes a single value, you simply add your new property to the ParseSingleProperty method. If a more complex parsing is required you will have to write a new method to handle it, modeling it off of one of the existing parsing helper methods (see ParseBackground , for and example). We are just adding a simple single-value property here.

Open nsCSSParser.cpp and look for the method ParseSingleProperty . This method is responsible for calling the relevant helper routine to parse the value(s). Find an existing property that is similar to the property you are adding. For our example we are adding a property that takes a numeric value so we will model it after the 'height' property and call ParsePositiveVariant. Add a new case for the new property and call the appropriate parser-helper and make a call to ParseVariant passing the variant flag that makes sense for your property. In our case

  case eCSSProperty_force_broken_image_icons:
    return ParsePositiveVariant(aErrorCode, aValue, VARIANT_INTEGER, nullptr);
This will parse the value as a positive integer value, which is what we want.

Style Context

Having implemented support for the new property in the CSS Parser and CSS Declaration classes in the content module, it is now time to provide support for the new property in layout. The Style Context must be given a new data member corresponding to the declaration's new data member, so the computed value can be held for the layout objects to use.

First look into nsStyleStruct.h to see the existing style strucs. Find the one that you want to store the data on. In this example, we want to put it on the nsStyleUI struct, however there is also a class nsStyleUIReset that holds the non-inherited values, so we will use that one (remember, our property is not inherited). Add a data member to hold the value:
struct nsStyleUIReset {
  nsStyleUIReset(const nsStyleUIReset& aOther);


  void* operator new(size_t sz, nsPresContext* aContext) {
    return aContext->AllocateFromShell(sz);
  void Destroy(nsPresContext* aContext) {
    aContext->FreeToShell(sizeof(nsStyleUIReset), this);

  int32_t CalcDifference(const nsStyleUIReset& aOther) const;

  uint8_t   mUserSelect;      // [reset] (selection-style)
  PRUnichar mKeyEquivalent;   // [reset] XXX what type should this be?
  uint8_t   mResizer;         // [reset]
  uint8_t   mForceBrokenImageIcon; // [reset]  (0 if not forcing, otherwise forcing)
In the implementation file ComputedStyle.cpp add the new data member to the constructors of the style struct and the CalcDifference method, which must return the correct style-change hint when a change to your new property is detected. The constructor changes are obvious, but here is the CalcDifference change for our example:
int32_t nsStyleUIReset::CalcDifference(const nsStyleUIReset& aOther) const
  if (mForceBrokenImageIcon == aOther.mForceBrokenImageIcon) {
    if (mResizer == aOther.mResizer) {
      if (mUserSelect == aOther.mUserSelect) {
        if (mKeyEquivalent == aOther.mKeyEquivalent) {
          return NS_STYLE_HINT_NONE;
        return NS_STYLE_HINT_CONTENT;
      return NS_STYLE_HINT_VISUAL;
  return nsChangeHint_ReconstructFrame;


The nsCSSStyleRule must be updated to manage mapping the declaration to the style struct. In the file nsCSSStyleRule.cpp , locate the Declaration mapping function corresponding to the style struct you have added your property to. For example, we update MapUIForDeclaration:
static nsresult
MapUIForDeclaration(nsCSSDeclaration* aDecl, const nsStyleStructID& aID, nsCSSUserInterface& aUI)
  if (!aDecl)
    return NS_OK; // The rule must have a declaration.

  nsCSSUserInterface* ourUI = (nsCSSUserInterface*)aDecl->GetData(kCSSUserInterfaceSID);
  if (!ourUI)
    return NS_OK; // We don't have any rules for UI.

  if (aID == eStyleStruct_UserInterface) {
    if (aUI.mUserFocus.GetUnit() == eCSSUnit_Null && ourUI->mUserFocus.GetUnit() != eCSSUnit_Null)
      aUI.mUserFocus = ourUI->mUserFocus;
    if (aUI.mUserInput.GetUnit() == eCSSUnit_Null && ourUI->mUserInput.GetUnit() != eCSSUnit_Null)
      aUI.mUserInput = ourUI->mUserInput;

    if (aUI.mUserModify.GetUnit() == eCSSUnit_Null && ourUI->mUserModify.GetUnit() != eCSSUnit_Null)
      aUI.mUserModify = ourUI->mUserModify;

    if (!aUI.mCursor && ourUI->mCursor)
      aUI.mCursor = ourUI->mCursor;

  else if (aID == eStyleStruct_UIReset) {
    if (aUI.mUserSelect.GetUnit() == eCSSUnit_Null && ourUI->mUserSelect.GetUnit() != eCSSUnit_Null)
      aUI.mUserSelect = ourUI->mUserSelect;
    if (!aUI.mKeyEquivalent && ourUI->mKeyEquivalent)
      aUI.mKeyEquivalent = ourUI->mKeyEquivalent;

    if (aUI.mResizer.GetUnit() == eCSSUnit_Null && ourUI->mResizer.GetUnit() != eCSSUnit_Null)
      aUI.mResizer = ourUI->mResizer;
    if (aUI.mForceBrokenImageIcon.GetUnit() == eCSSUnit_Null && ourUI->mForceBrokenImageIcon.GetUnit() == eCSSUnit_Integer)
      aUI.mForceBrokenImageIcon = ourUI->mForceBrokenImageIcon;

  return NS_OK;


Rule Node

Now we have to update the RuleNode code to know about the new property. First, locate the PropertyCheckData array for the data that you added the new property to. For this example, we add the following:
static const PropertyCheckData UIResetCheckProperties[] = {
  CHECKDATA_PROP(nsCSSUserInterface, mForceBrokenImageIcon, CHECKDATA_VALUE, PR_FALSE)
The first two arguments correspond to the structure and data member from the CSSDeclaration, the third is the data type, the fourth indicates whether it is a coord value that uses an explicit inherit value on the style data struct that must be computed by layout.

Next, we have to make sure the ComputeXXX method for the structure the property was added to is updated to mange the new value. In this example we need to modify the nsRuleNode::ComputeUIResetData method to handle the CSS Declaration to the style struct:
  // resizer: auto, none, enum, inherit
  if (eCSSUnit_Enumerated == uiData.mResizer.GetUnit()) {
    ui->mResizer = uiData.mResizer.GetIntValue();
  else if (eCSSUnit_Auto == uiData.mResizer.GetUnit()) {
    ui->mResizer = NS_STYLE_RESIZER_AUTO;
  else if (eCSSUnit_None == uiData.mResizer.GetUnit()) {
    ui->mResizer = NS_STYLE_RESIZER_NONE;
  else if (eCSSUnit_Inherit == uiData.mResizer.GetUnit()) {
    inherited = PR_TRUE;
    ui->mResizer = parentUI->mResizer;

  // force-broken-image-icons: integer, inherit, initial
  if (eCSSUnit_Integer == uiData.mForceBrokenImageIcons.GetUnit()) {
    ui->mForceBrokenImageIcons = uiData.mForceBrokenImageIcons.GetIntValue();
  } else if (eCSSUnit_Inherit == uiData.mForceBrokenImageIcons.GetUnit()) {
    inherited = PR_TRUE;
    ui->mForceBrokenImageIcons = parentUI->mForceBrokenImageIcons;
  } else if (eCSSUnit_Initial == uiData.mForceBrokenImageIcons.GetUnit()) {
    ui->mForceBrokenImageIcons = 0;
  if (inherited)
    // We inherited, and therefore can't be cached in the rule node.  We have to be put right on the
    // style context.
    aContext->SetStyle(eStyleStruct_UIReset, *ui);
  else {
    // We were fully specified and can therefore be cached right on the rule node.
    if (!aHighestNode->mStyleData.mResetData)
      aHighestNode->mStyleData.mResetData = new (mPresContext) nsResetStyleData;
    aHighestNode->mStyleData.mResetData->mUIData = ui;
    // Propagate the bit down.
    PropagateDependentBit(NS_STYLE_INHERIT_UI_RESET, aHighestNode);


Users in scripts, or anywhere outside of layout/ or content/ may need to access the new property. This is done using the CSS OM, specifically nsIDOMCSSStyleDeclaration and CSS2Properties. By the magic of C++ pre-processing, the CSS2Properties bits will be implemented automatically when you add your property to nsCSSPropList.h.


OK, finally the style system is supporting the new property. It is time to actually make use of it now.

In layout, retrieve the styleStruct that has the new property from the frame's style context. Access the new property and get its value. It is that simple. For this example, it looks like this, in nsImageFrame:
        PRBool forceIcon = PR_FALSE;

        if (StyleUIReset()->mForceBrokenImageIcon) {
          forceIcon = PR_TRUE;

Create some testcases with style rules that use the new property, make sure it is being parsed correctly. Test it in an external stylesheet and in inline style. Test that it is inherited correctly, or not inherited as appropriate to your property. Update this document with any further details, or correcting any errors.