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+# Open/Close Expressions
+
+Status: **Proposed**
+
+<details>
+	<summary>Metadata</summary>
+	<dl>
+		<dt>Contributors</dt>
+		<dd>@eemeli</dd>
+		<dd>@aphillips</dd>
+		<dt>First proposed</dt>
+		<dd>2023-09-05</dd>
+		<dt>Pull Request</dt>
+		<dd><a href="https://github.com/unicode-org/message-format-wg/pull/470">#470</a></dd>
+	</dl>
+</details>
+
+## Objective
+
+_What is this proposal trying to achieve?_
+
+Describe the use cases and requirements for _expressions_
+that enclose parts of a pattern
+and develop a design that satisfies these needs.
+
+## Background
+
+_What context is helpful to understand this proposal?_
+
+## Use-Cases
+
+- Representing markup, such as HTML, as expressions in a pattern
+  rather than as plain character sequences embedded in the _text_
+  of a pattern. This also allows parts of a markup sequence to be
+  programmable.
+
+- As an app author, I want to be able to use minimal markup for formatting.
+  On runtime, I expect these markup elements to produce live UI elements.
+
+  > ```
+  > {Click {+link}here{-link}.}
+  > ```
+
+- As an app author, I want to be able to interpolate standalone markup elements into the translation: `img`, `hr`, `input`.
+  On runtime, I expect these markup elements to produce live UI elements.
+
+  > ```
+  > {This is a giraffe: {#img src=giraffe.gif}.}
+  > ```
+
+- I want to be able to use minimal markup to inform XLIFF interchange.
+
+  > ```
+  > {Click {+ph}<a href="">{-ph}here{+ph}</a>{-ph}.}
+  > ```
+
+- As an app author, I want to be able to pass certain attributes to markup elements, including dynamic values, such as coming from variables.
+
+  > ```
+  > {Click {+link href=$url}here{-link}.}
+  > ```
+
+- As a translator, I want to be able to translate content around and between markup elements.
+
+  > ```
+  > {Kliknij {+link}tutaj{-link}.}
+  > ```
+
+- As a translator, I want to be able to translate certain markup attributes.
+
+  > ```
+  > {Click {+link title=|Hey you!|}here{-link}}
+  > ```
+
+- As a developer or as a translator, I want to protect placeholders from modification and deletion.
+
+- Paired markup elements should be protected from reordering, i.e. an open element must always come before the close element (if they are both present in the message).
+
+- As a developer working in certain domains, I want to be able to use markup such as TTML or SSML.
+
+- As a developer, I want to be able to produce "attributed strings" appropriate for my environment, which may include features such as overlapping or enclosing properties.
+
+- As a developer in certain environments, I want to be able to include TTY formatting escapes (?!) in my strings.
+
+- As a developer, I want to be able to split up text between strings, such as open or close elements are in separate strings.
+- As a translator or tool, I want to be able to segment larger messages or process messages "segmented" from larger blocs of text, such that markup is split between messages or segments.
+
+- As a translator, I want my tools to be able to leverage translations where the text differs only in markup, e.g.:
+
+  > ```
+  > {This is {+b}sure{-b} good.}
+  > {This is {+i}sure{-i}good.}
+  > ```
+
+- As a CAT tool, I want to use the concepts of open, close, and standalone that I am already familiar with, to provide certain functionalities above.
+
+- As a developer, I want to connect message markup with existing element instances in the UI.
+
+- As a developer, I want to sanitize translations without removing the markup important to the message.
+
+  > Source HTML:
+  >
+  > ```
+  > <popup-info img="img/alt.png" data-text="Your card validation code (CVC) is an extra security feature — it is the last 3 or 4 numbers on the back of your card."></popup-info>
+  > ```
+  >
+  > Message:
+  >
+  > ```
+  > {+popup-info img=|img/alt.png| data-text=|Your card validation code (CVC) is an extra security feature — it is the last 3 or 4 numbers on the back of your card.|}{-popup-info}
+  > ```
+
+**_Non-markup use cases to consider (which may or may not be addressed by the design)_**
+
+- Represent templating language constructs so that they can pass through
+  the translation process and are not exposed to translation. For example,
+  Mustache templates.
+
+- Represent translation metadata, such as XLIFF placeholders, so that
+  portions of a message are "protected" or otherwise hinted in the
+  translation process.
+
+## Requirements
+
+Be able to indicate that some identified markup applies to
+a non-empty sequence of pattern parts (text, expressions).
+
+Markup spans may be nested,
+as in `<b>Bold and <i>also italic</i></b>`.
+
+Markup may have localisable options,
+such as `Click <a title="Link tooltip">here</a> to continue`.
+
+As with function options,
+markup options should be defined in a registry so that they can be validated.
+
+## Constraints
+
+Due to segmentation,
+markup may be split across messages so that it opens in one and closes in another.
+
+Following previously established consensus,
+the resolution of the value of an _expression_ may only depend on its own contents,
+without access to the other parts of the selected pattern.
+
+## Proposed Design
+
+This design relies on the recognition that the formatted output of MF2
+may be further processed by other tools before presentation to a user.
+
+Let us add _markup_ as a new type of _placeholder_,
+in parallel with _expression_:
+
+```abnf
+pattern = "{" *(text / placeholder) "}"
+placeholder = expression / markup
+
+markup       = "{" [s] markup-body [s] "}"
+markup-body  = (markup-standalone *(s option))
+             / (markup-open *(s option))
+             / markup-close
+markup-standalone = "#" name
+markup-open       = "+" name
+markup-close      = "-" name
+```
+
+This allows for placeholders like `{+b}`, `{#img}`, and `{+a title=|Link tooltip|}`.
+Unlike annotations, markup expressions may not have operands.
+
+Markup is not valid in _declarations_ or _selectors_.
+
+When formatting to a string,
+markup placholders format to an empty string by default.
+An implementation may customize this behaviour,
+e.g. emitting XML-ish tags for each open/close placeholder.
+
+When formatting to parts (as proposed in <a href="https://github.com/unicode-org/message-format-wg/pull/463">#463</a>),
+markup placeholders format to an object including the following properties:
+
+- The `type` of the markup: `"open" | "close" | "standalone"`
+- The `name` of the markup, e.g. `"b"` for `{+b}`
+- For _markup-open_ and _markup-standalone_,
+  the `options` with the resolved key-value pairs of the expression options
+
+To make use of _markup_,
+the message should be formatted to parts or to some other target supported by the implementation,
+and the desired shape constructed from the parts.
+For example, the message
+
+```
+{Click {+a title=|Link tooltip|}here{-a} to continue}
+```
+
+would format to parts as
+
+```coffee
+[
+  { type: "text", value: "Click " },
+  { type: "open", name: "a", options: { title: "Link tooltip" } },
+  { type: "text", value: "here" },
+  { type: "close", name: "a" },
+  { type: "text", value: " to continue" }
+]
+```
+
+and a post-MessageFormat step could then reconstruct this into an appropriate shape,
+potentially merging in non-localizable attributes such as a link URL.
+Rendered as React this could become:
+
+```coffee
+[
+  "Click ",
+  createElement("a", { href: "/next", title: "Link tooltip" }, "here"),
+  " to continue"
+]
+```
+
+## Alternatives Considered
+
+_What other solutions are available?_
+_How do they compare against the requirements?_
+_What other properties they have?_