diff --git a/app/test/firehose_web/components/core_components_test.exs b/app/test/firehose_web/components/core_components_test.exs
new file mode 100644
index 0000000..dc3d46e
--- /dev/null
+++ b/app/test/firehose_web/components/core_components_test.exs
@@ -0,0 +1,226 @@
+defmodule FirehoseWeb.CoreComponentsTest do
+  # Pure render tests: no conn, no database, so plain ExUnit.Case (the same
+  # choice layouts_test.exs makes) instead of the ConnCase sandbox.
+  use ExUnit.Case, async: true
+
+  import Phoenix.LiveViewTest
+
+  alias FirehoseWeb.CoreComponents
+
+  @checkbox_assigns %{
+    type: "checkbox",
+    name: "prefs[news]",
+    id: "prefs_news",
+    label: "News",
+    value: "true",
+    rest: %{},
+    errors: [],
+    field: nil
+  }
+
+  @select_assigns %{
+    type: "select",
+    name: "role",
+    id: "role",
+    label: "Role",
+    prompt: "Pick one",
+    options: [{"Admin", "admin"}, {"Editor", "editor"}],
+    value: "admin",
+    multiple: false,
+    rest: %{},
+    errors: [],
+    field: nil
+  }
+
+  @textarea_assigns %{
+    type: "textarea",
+    name: "body",
+    id: "body",
+    label: "Body",
+    value: "hello world",
+    rest: %{},
+    errors: [],
+    field: nil
+  }
+
+  describe "input/1 checkbox" do
+    test "a checked box renders checked when the value is true" do
+      html = render_checkbox(%{})
+
+      assert attr(html, "input[type='checkbox']", "name") == "prefs[news]"
+      assert attr(html, "input[type='checkbox']", "value") == "true"
+      assert attr(html, "input[type='checkbox']", "checked") == ""
+      assert text(html) =~ "News"
+    end
+
+    test "an unchecked box still posts false for the field" do
+      html = render_checkbox(%{value: "false"})
+
+      assert attr(html, "input[type='checkbox']", "checked") == nil
+
+      # Markup is the behaviour here: the hidden field carrying the same name is
+      # what makes an unchecked box post "false" instead of posting nothing.
+      assert attr(html, "input[type='hidden']", "name") == "prefs[news]"
+      assert attr(html, "input[type='hidden']", "value") == "false"
+    end
+
+    test "shows the error message when the field has errors" do
+      html = render_checkbox(%{errors: ["can't be blank"]})
+
+      assert text(html) =~ "can't be blank"
+    end
+  end
+
+  describe "input/1 select" do
+    test "renders the prompt, every option, and preselects the current value" do
+      html = render_select(%{})
+
+      assert text(html) =~ "Pick one"
+      assert text(html) =~ "Admin"
+      assert text(html) =~ "Editor"
+      # The outcome, not the serialisation: the option carrying the current
+      # value is selected, the other one is not.
+      assert attr(html, "option[value='admin']", "selected") == ""
+      assert attr(html, "option[value='editor']", "selected") == nil
+    end
+
+    test "marks the select erroneous and shows the message when given errors" do
+      html = render_select(%{errors: ["is invalid"]})
+
+      assert text(html) =~ "is invalid"
+      assert "select-error" in classes(html, "select")
+    end
+  end
+
+  describe "input/1 textarea" do
+    test "keeps the value inside the textarea" do
+      html = render_textarea(%{value: "hello world"})
+
+      # normalize_value/2 prefixes the content with a newline, so compare the
+      # trimmed text: what matters is the value, not the surrounding whitespace.
+      assert String.trim(text(html, "textarea")) == "hello world"
+    end
+
+    test "renders an empty textarea for a nil value instead of the text nil" do
+      html = render_textarea(%{value: nil})
+
+      assert String.trim(text(html, "textarea")) == ""
+      refute html =~ "nil"
+    end
+
+    test "keeps a leading newline in the value" do
+      # HTML drops one leading newline of a <textarea> before the browser sees
+      # it, so normalize_value/2 prefixes the content with one: a value that
+      # starts with a newline survives, and one that does not gain one.
+      html = render_textarea(%{value: "\nsecond line"})
+
+      assert text(html, "textarea") == "\nsecond line"
+    end
+
+    test "marks the textarea erroneous and shows the message when given errors" do
+      html = render_textarea(%{errors: ["is too short"]})
+
+      assert text(html) =~ "is too short"
+      assert "textarea-error" in classes(html, "textarea")
+    end
+  end
+
+  describe "button/1" do
+    test "renders a button element when given no destination" do
+      html = render_button(%{})
+
+      assert count(html, "button") == 1
+      assert count(html, "a") == 0
+      assert String.trim(text(html)) == "Save"
+    end
+
+    test "renders a link to the destination when given an href, keeping the label" do
+      html = render_button(%{rest: %{href: "/somewhere"}})
+
+      assert attr(html, "a", "href") == "/somewhere"
+      assert String.trim(text(html)) == "Save"
+      assert count(html, "button") == 0
+    end
+
+    test "renders a live-navigation link when given navigate" do
+      html = render_button(%{rest: %{navigate: "/elsewhere"}})
+
+      # data-phx-link is the only thing distinguishing a live-navigation link
+      # from a full-page one, so it is the behaviour to pin here.
+      assert attr(html, "a", "data-phx-link") == "redirect"
+      assert attr(html, "a", "href") == "/elsewhere"
+    end
+  end
+
+  defp render_checkbox(overrides) do
+    render_component(&CoreComponents.input/1, Map.merge(@checkbox_assigns, overrides))
+  end
+
+  defp render_select(overrides) do
+    render_component(&CoreComponents.input/1, Map.merge(@select_assigns, overrides))
+  end
+
+  defp render_textarea(overrides) do
+    render_component(&CoreComponents.input/1, Map.merge(@textarea_assigns, overrides))
+  end
+
+  defp render_button(overrides) do
+    assigns =
+      Map.merge(
+        %{
+          variant: nil,
+          class: "",
+          rest: %{},
+          # A slot, as HEEx builds it: a list of entries whose inner_block takes
+          # (argument, changed).
+          inner_block: [%{inner_block: fn _, _ -> "Save" end}]
+        },
+        overrides
+      )
+
+    render_component(&CoreComponents.button/1, assigns)
+  end
+
+  # Every attribute assertion goes through here, so the LazyHTML mechanism lives
+  # in one place: one value per matched element, nil when the element (or the
+  # attribute) is absent, "" for a valueless attribute such as `checked`.
+  defp attr(html, selector, name) do
+    html
+    |> nodes(selector)
+    |> LazyHTML.attribute(name)
+    |> List.first()
+  end
+
+  # The class tokens of every matched element, for the "this field is marked
+  # erroneous" assertions.
+  defp classes(html, selector) do
+    html
+    |> nodes(selector)
+    |> LazyHTML.attribute("class")
+    |> Enum.flat_map(&String.split/1)
+  end
+
+  defp nodes(html, selector) do
+    html
+    |> LazyHTML.from_fragment()
+    |> LazyHTML.query(selector)
+  end
+
+  defp count(html, selector) do
+    html
+    |> nodes(selector)
+    |> Enum.count()
+  end
+
+  defp text(html) do
+    html
+    |> LazyHTML.from_fragment()
+    |> LazyHTML.text()
+  end
+
+  defp text(html, selector) do
+    html
+    |> nodes(selector)
+    |> LazyHTML.text()
+  end
+end