HTML Imports

The Source/core/html/imports directory contains the implementation of HTML Imports.

The spec can be found here.

Basic Data Structure and Algorithms of HTML Imports implementation.

The Import Tree

HTML Imports form a tree:

• The root of the tree is HTMLImportTreeRoot.
• The HTMLImportTreeRoot is owned by HTMLImportsController, which is owned by the tree_root document as a DocumentSupplement.
• The non-root nodes are HTMLImportChild. They are all owned by HTMLImporTreeRoot. LinkStyle is wired into HTMLImportChild by implementing HTMLImportChildClient interface.
• Both HTMLImportTreeRoot and HTMLImportChild are derived from HTMLImport superclass that models the tree data structure using WTF::TreeNode and provides a set of virtual functions.

HTMLImportsController also owns all loaders in the tree and manages their lifetime through it. One assumption is that the tree is append-only and nodes are never inserted in the middle of the tree nor removed.

Full diagram is here

Import Sharing and HTMLImportLoader

The HTML Imports spec calls for de-dup mechanism to share already loaded imports. To implement this, the actual loading machinery is split out from HTMLImportChild to HTMLImportLoader, and each loader shares HTMLImportLoader with other loader if the URL is same. Check around HTMLImportTreeRoot::Find() for more detail.

HTMLImportLoader can be shared by multiple imports.

   HTMLImportChild (1)-->(*) HTMLImportLoader

Script Blocking

• An import blocks the HTML parser of its own imported document from running <script> until all of its children are loaded. Note that dynamically added import won't block the parser.
• An import under loading also blocks imported documents that follow from being created. This is because an import can include another import that has same URLs of following ones. In such case, the preceding import should be loaded and following ones should be de-duped.