diff --git a/src/openflexure_microscope_server/ui.py b/src/openflexure_microscope_server/ui.py
index 3cb4375e..f24726d3 100644
--- a/src/openflexure_microscope_server/ui.py
+++ b/src/openflexure_microscope_server/ui.py
@@ -1,11 +1,127 @@
 """Functionality for communicating the required user interface for a thing."""
 
-from typing import Any, Optional
+from html import escape
+from html.parser import HTMLParser
+from typing import Annotated, Any, Literal, Optional
 
-from pydantic import BaseModel
+from pydantic import BaseModel, BeforeValidator
 
 import labthings_fastapi as lt
 
+ALLOWED_TAGS = {
+    "p",
+    "i",
+    "b",
+    "h1",
+    "h2",
+    "h3",
+    "h4",
+    "h5",
+    "h6",
+    "ul",
+    "li",
+    "a",
+}
+
+
+class SafeHTMLParser(HTMLParser):
+    """An HTMLParser that only allows a very limited subset of HTML."""
+
+    def __init__(self) -> None:
+        """Initialise the parser."""
+        super().__init__()
+        self.output = ""
+
+    def handle_starttag(self, tag: str, attrs: dict[str, str]) -> None:
+        """Append a start tag found in the HTML if it is in the allowed list.
+
+        This is called by the base class when a start tag is found. We only append
+        the tag if it is our allowed list. We re-write the tag so no extra attributes
+        can be added.
+
+        No tags can have attributes except for ``a`` tags only allows ``href``. We then
+        append ``target="_blank" rel="noopener noreferrer"`` to ensure the link opens
+        in a new window.
+
+        :param tag: the tag name
+        :param attrs: the attributes for the tag.
+        """
+        if tag not in ALLOWED_TAGS:
+            return
+
+        if tag == "a":
+            href = None
+            for attr, value in attrs:
+                if attr == "href":
+                    href = value.strip()
+
+            if href:
+                safe_href = escape(href, quote=True)
+                # Always in a new tab
+                self.output.append(
+                    f'<a href="{safe_href}" target="_blank" rel="noopener noreferrer">'
+                )
+            else:
+                self.output += "<a>"
+        else:
+            self.output += f"<{tag}>"
+
+    def handle_endtag(self, tag: str) -> None:
+        """Append the end tag only if it is in the allowed list."""
+        if tag in ALLOWED_TAGS:
+            self.output += f"</{tag}>"
+
+    def handle_startendtag(self, tag: str, _attrs: dict[str, str]) -> None:
+        """Append a self-closing tag if it is a new line."""
+        if tag == "br":
+            self.output += "<br/>"
+
+    def handle_data(self, data: str) -> None:
+        """Append any data inside tags after escaping it."""
+        self.output += escape(data)
+
+    def handle_entityref(self, name: str) -> None:
+        """Append any named character."""
+        self.output += f"&{name};"
+
+    def handle_charref(self, name: str) -> None:
+        """Append any character references."""
+        self.output += f"&#{name};"
+
+
+def sanitize_html(html: str) -> str:
+    """Santitise HTML to only have a small list of allowed tags.
+
+    Tags allowed without attrs: ``<p>, <i>, <b>, <h1>, <h2>, <h3>, <h4>, <h5>, <h6>,``
+    ``<ul>, <li>``
+
+    Tags allowed with attrs: ``<a>`` is allowed with ``href`` only. This automatically
+    appends ``target="_blank" rel="noopener noreferrer"`` so the link opens externally.
+
+    Self closing tags allowed ``<br>``.
+
+    :param html: The input HTML.
+
+    :return: The sanitised HTML.
+    """
+    parser = SafeHTMLParser()
+    parser.feed(html)
+    parser.close()
+    return parser.output
+
+
+HtmlFragment = Annotated[str, BeforeValidator(sanitize_html)]
+
+
+class HTMLBlock(BaseModel):
+    """The data required for creating a block of HTML for the UI.
+
+    The HTML is limited to a small number of tags.
+    """
+
+    element_type: Literal["html_block"] = "html_block"
+    html: HtmlFragment
+
 
 class ActionButton(BaseModel):
     """The data required for creating an actionButton in Vue.
@@ -13,6 +129,8 @@ class ActionButton(BaseModel):
     Currently this cannot be used to submitData, or to submitOnEvent.
     """
 
+    element_type: Literal["action_button"] = "action_button"
+
     thing: str
     """The Thing "path" for the Thing instance."""
 
@@ -72,6 +190,8 @@ class PropertyControl(BaseModel):
     Currently this cannot be used to submitData, or to submitOnEvent.
     """
 
+    element_type: Literal["property_control"] = "property_control"
+
     thing: str
     """The Thing "path" for the Thing instance."""
 
@@ -121,3 +241,33 @@ def property_control_for(
     if "label" not in kwargs:
         kwargs["label"] = property_name
     return PropertyControl(thing=thing.name, property_name=property_name, **kwargs)
+
+
+class Accordion(BaseModel):
+    """The data required for creating an accordion in the UI.
+
+    The HTML is limited to a small number of tags.
+    """
+
+    element_type: Literal["accordion"] = "accordion"
+    title: str
+    children: list["UIElementModels"]
+
+
+class Container(BaseModel):
+    """The data required for creating ``<div>`` in the UI.
+
+    The HTML is limited to a small number of tags.
+    """
+
+    element_type: Literal["container"] = "container"
+    css_class: str
+    children: list["UIElementModels"]
+
+
+UIElementModels = HTMLBlock | PropertyControl | ActionButton | Accordion | Container
+
+
+# Resolve forward references so Pydantic can build a sensible recursive schema
+Accordion.model_rebuild()
+Container.model_rebuild()