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Component Model Discussion

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This document attempts to exhaustively list issues that we feel are unanswered or ambiguous.

Linked pages:


CSS and Rendering

Other issues:

  • styling of rendering children of <content> (partially discussed in the CSS brainstorming page).
  • Do components create a layer?
    • assumption: no
Dglazkov isn't this an implementation detail?
  • tab-index behavior
  • z-order behavior

Shadow Nodes

  • shadow root being a separate node type vs. an element vs. document fragment vs. ??? (see rendering sub-page)
    • current state: separate node type
  • Does ShadowRoot have innerHTML?
    • assumption: yes
  • details of <content> selector syntax
    • suggestion: space-separated list of selector sequences: [tag]?[class]*[id]*[attr]*[pseudo-class]* (NO pseudo-elements)
Dglazkov will space-separation be confusing? I like the idea, but just thought I should raise this as a concern.
  • Can the same host element child be rendered under multiple <content> elements that all select it?
    • assumption: no - the first to select it "gets" it. <content> without select is always last in this regard
  • Is <content> is a bona-fide HTML element (vs. a DOM subtype)?
    • assumption: HTML element


  • focus event traversing out of shadow?
    • assumption: yes, redirected
    • current state: not implemented
  • instantiation events (which events & when?)
    • for <template> instantiations
    • for component instances
    • on rebinding? (provided CSS 'binding' is possible)
  • event on component registration?

Loading and Life Cycle

  • loading mechanism (syntax)
  • How to handle definition after use
    • assumption: don't do anything
  • packaging
  • is it possible to rename a component? e.g., to avoid name clashes?
    • assumption: no
  • is it possible to have a scoped (private) names, e.g., for private components?
    • assumption: no
  • define all edge cases and “unhappy” cases. Examples: constructor throws exception (general case), constructor throws exception during parsing
  • Is it possible to undefine a component? (if yes, how?)
    • assumption: no


  • how to handle name clashes between components?
    • assumption: teach authors to add unique prefix, e.g., <x-cyberdyne-terminator>
  • details on attaching a shadow tree?
    • <x-mywidget>
      • Pros: flexible, tag is inherently static, most performant
      • Cons: accessibility, fall-back
        • <div becomes=mywidget> even more problematic in these regards and also requires duplication of style rules (?)
    • <button is=mywidget>
      • Pros: accessibility of base element, natural fallback
      • Cons: not a priori obvious that 'is' cannot be rebound, limited to existing element names
    • CSS binding: button { binding: mywidget; }
      • Pros: accessibility and fallback, most flexible
      • Cons: implementation is hard, multiple (perhaps mutually exclusive) bindings may apply, cannot extend the API
    • other?
  • fallback mechanisms (implicit with <button is=mywidget>, explicit, e.g., with fallback content as children)
  • Are shadow trees dynamic or static?
    • assumption: static, but see discussion in public-webapps@ ML
  • accessibility (implicit, e.g. with <button is=mywidget>, explicit, e.g., with ARIA roles)


  • should we do XBL-like inheritance?
    • assumption: no
  • “aspect-oriented" components (components defining the parts they use, derived components can override or re-use specific parts, part names automatically become pseudos)
  • composition vs. inheritance (i.e., do we need inheritance at all, or can composition handle all use cases?)
    • temporary assumption: composition only


Subtyping of DOM Objects

  • Can one subtype anything (text/attribute nodes, events, ...), or just HTMLElement?
    • assumption: just HTML element
  • details of construction
  • can one write a constructor with <supertype>.call(...), or does one have to use createElement?


  • JS implementation details


examples: sub-classing <textarea> (Maciej’s examples), auto-completion, omni-bar like drop-down, toolbar

  • JS implementation details


  • how to handle Element.register being overwritten?
    • assumption: don't care
  • how to handle, e.g., appendChild being overwritten on a component?
    • assumption: don't care

Isolation and Confinement

(see also the isolation brainstorming page)

  • To what degree is isolation/confinement really necessary? (scripts? DOM access? CSS properties?)
  • How far is isolation/confinement handling for components similar to <iframe>?
  • Can we substitute isolation with <iframe>s?
  • Are instances of the same component isolated from each other?
  • Are instances of the components from the same domain isolated from each other?
  • Can instances of the same component share data somehow?
  • Can instances of the same component and its derivatives share data somehow?
  • Can instances of the components from the same domain share data somehow?
  • Can isolated components position content outside the host boundary rect? (if not, problems with, e.g., +1 widget)
    • assumption: yes

Declarative syntax

  • how/when to run scripts?
  • syntax to style the host (provided we select a model where it is rendered)?
  • <template> details:
    • usable without components?
    • instantiable without explicit components (e.g., <instance select=”x-cyberdyne-terminator”>)
    • event handler registration syntax
    • introduce template types (e.g., <template type=”django”>)?
      • assumption: no
  • <element> details
  • Do we need <binding> as well?
  • How to define the API of components


  • Do we define the shadow DOM (or parts thereof) of built-in elements?
    • assumption: no
  • How can we split the spec into (more) different parts?