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-rw-r--r--test/unit/util/html.js934
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diff --git a/test/unit/util/html.js b/test/unit/util/html.js
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+import t from 'tap';
+
+import * as html from '../../../src/util/html.js';
+const {Tag, Attributes, Template} = html;
+
+import {strictlyThrows} from '../../lib/strict-match-error.js';
+
+t.test(`html.tag`, t => {
+  t.plan(14);
+
+  const tag1 =
+    html.tag('div',
+      {[html.onlyIfContent]: true, foo: 'bar'},
+      'child');
+
+  // 1-5: basic behavior when passing attributes
+  t.ok(tag1 instanceof Tag);
+  t.ok(tag1.onlyIfContent);
+  t.equal(tag1.attributes.get('foo'), 'bar');
+  t.equal(tag1.content.length, 1);
+  t.equal(tag1.content[0], 'child');
+
+  const tag2 = html.tag('div', ['two', 'children']);
+
+  // 6-8: basic behavior when not passing attributes
+  t.equal(tag2.content.length, 2);
+  t.equal(tag2.content[0], 'two');
+  t.equal(tag2.content[1], 'children');
+
+  const genericTag = html.tag('div');
+  const genericTemplate = html.template({
+    content: () => html.blank(),
+  });
+
+  // 9-10: tag treated as content, not attributes
+  const tag3 = html.tag('div', genericTag);
+  t.equal(tag3.content.length, 1);
+  t.equal(tag3.content[0], genericTag);
+
+  // 11-12: template treated as content, not attributes
+  const tag4 = html.tag('div', genericTemplate);
+  t.equal(tag4.content.length, 1);
+  t.equal(tag4.content[0], genericTemplate);
+
+  // 13-14: deep flattening support
+  const tag6 =
+    html.tag('div', [
+      true &&
+        [[[[[[
+          true &&
+            [[[[[`That's deep.`]]]]],
+        ]]]]]],
+    ]);
+  t.equal(tag6.content.length, 1);
+  t.equal(tag6.content[0], `That's deep.`);
+});
+
+t.test(`Tag (basic interface)`, t => {
+  t.plan(11);
+
+  const tag1 = new Tag();
+
+  // 1-5: essential properties & no arguments provided
+  t.equal(tag1.tagName, '');
+  t.ok(Array.isArray(tag1.content));
+  t.equal(tag1.content.length, 0);
+  t.ok(tag1.attributes instanceof Attributes);
+  t.equal(tag1.attributes.toString(), '');
+
+  const tag2 = new Tag('div', {id: 'banana'}, ['one', 'two', tag1]);
+
+  // 6-11: properties on basic usage
+  t.equal(tag2.tagName, 'div');
+  t.equal(tag2.content.length, 3);
+  t.equal(tag2.content[0], 'one');
+  t.equal(tag2.content[1], 'two');
+  t.equal(tag2.content[2], tag1);
+  t.equal(tag2.attributes.get('id'), 'banana');
+});
+
+t.test(`Tag (self-closing)`, t => {
+  t.plan(10);
+
+  const tag1 = new Tag('br');
+  const tag2 = new Tag('div');
+  const tag3 = new Tag('div');
+  tag3.tagName = 'br';
+
+  // 1-3: selfClosing depends on tagName
+  t.ok(tag1.selfClosing);
+  t.notOk(tag2.selfClosing);
+  t.ok(tag3.selfClosing);
+
+  // 4: constructing self-closing tag with content throws
+  t.throws(() => new Tag('br', null, 'bananas'), /self-closing/);
+
+  // 5: setting content on self-closing tag throws
+  t.throws(() => { tag1.content = ['suspicious']; }, /self-closing/);
+
+  // 6-9: setting empty content on self-closing tag doesn't throw
+  t.doesNotThrow(() => { tag1.content = null; });
+  t.doesNotThrow(() => { tag1.content = undefined; });
+  t.doesNotThrow(() => { tag1.content = ''; });
+  t.doesNotThrow(() => { tag1.content = [null, '', false]; });
+
+  const tag4 = new Tag('div', null, 'bananas');
+
+  // 10: changing tagName to self-closing when tag has content throws
+  t.throws(() => { tag4.tagName = 'br'; }, /self-closing/);
+});
+
+t.test(`Tag (properties from attributes - from constructor)`, t => {
+  t.plan(6);
+
+  const tag = new Tag('div', {
+    [html.onlyIfContent]: true,
+    [html.noEdgeWhitespace]: true,
+    [html.joinChildren]: '<br>',
+  });
+
+  // 1-3: basic exposed properties from attributes in constructor
+  t.ok(tag.onlyIfContent);
+  t.ok(tag.noEdgeWhitespace);
+  t.equal(tag.joinChildren, '<br>');
+
+  // 4-6: property values stored on attributes with public symbols
+  t.equal(tag.attributes.get(html.onlyIfContent), true);
+  t.equal(tag.attributes.get(html.noEdgeWhitespace), true);
+  t.equal(tag.attributes.get(html.joinChildren), '<br>');
+});
+
+t.test(`Tag (properties from attributes - mutating)`, t => {
+  t.plan(12);
+
+  // 1-3: exposed properties reflect reasonable attribute values
+
+  const tag1 = new Tag('div', {
+    [html.onlyIfContent]: true,
+    [html.noEdgeWhitespace]: true,
+    [html.joinChildren]: '<br>',
+  });
+
+  tag1.attributes.set(html.onlyIfContent, false);
+  tag1.attributes.remove(html.noEdgeWhitespace);
+  tag1.attributes.set(html.joinChildren, '🍇');
+
+  t.equal(tag1.onlyIfContent, false);
+  t.equal(tag1.noEdgeWhitespace, false);
+  t.equal(tag1.joinChildren, '🍇');
+
+  // 4-6: exposed properties reflect unreasonable attribute values
+
+  const tag2 = new Tag('div', {
+    [html.onlyIfContent]: true,
+    [html.noEdgeWhitespace]: true,
+    [html.joinChildren]: '<br>',
+  });
+
+  tag2.attributes.set(html.onlyIfContent, '');
+  tag2.attributes.set(html.noEdgeWhitespace, 12345);
+  tag2.attributes.set(html.joinChildren, 0.0001);
+
+  t.equal(tag2.onlyIfContent, false);
+  t.equal(tag2.noEdgeWhitespace, true);
+  t.equal(tag2.joinChildren, '0.0001');
+
+  // 7-9: attribute values reflect reasonable mutated properties
+
+  const tag3 = new Tag('div', null, {
+    [html.onlyIfContent]: false,
+    [html.noEdgeWhitespace]: true,
+    [html.joinChildren]: '🍜',
+  })
+
+  tag3.onlyIfContent = true;
+  tag3.noEdgeWhitespace = false;
+  tag3.joinChildren = '🦑';
+
+  t.equal(tag3.attributes.get(html.onlyIfContent), true);
+  t.equal(tag3.attributes.get(html.noEdgeWhitespace), undefined);
+  t.equal(tag3.joinChildren, '🦑');
+
+  // 10-12: attribute values reflect unreasonable mutated properties
+
+  const tag4 = new Tag('div', null, {
+    [html.onlyIfContent]: false,
+    [html.noEdgeWhitespace]: true,
+    [html.joinChildren]: '🍜',
+  });
+
+  tag4.onlyIfContent = 'armadillo';
+  tag4.noEdgeWhitespace = 0;
+  tag4.joinChildren = Infinity;
+
+  t.equal(tag4.attributes.get(html.onlyIfContent), true);
+  t.equal(tag4.attributes.get(html.noEdgeWhitespace), undefined);
+  t.equal(tag4.attributes.get(html.joinChildren), 'Infinity');
+});
+
+t.test(`Tag.toString`, t => {
+  t.plan(9);
+
+  // 1: basic behavior
+
+  const tag1 =
+    html.tag('div', 'Content');
+
+  t.equal(tag1.toString(),
+    `<div>Content</div>`);
+
+  // 2: stringifies nested element
+
+  const tag2 =
+    html.tag('div', html.tag('p', 'Content'));
+
+  t.equal(tag2.toString(),
+    `<div><p>Content</p></div>`);
+
+  // 3: stringifies attributes
+
+  const tag3 =
+    html.tag('div',
+      {
+        id: 'banana',
+        class: ['foo', 'bar'],
+        contenteditable: true,
+        biggerthanabreadbox: false,
+        saying: `"To light a candle is to cast a shadow..."`,
+        tabindex: 413,
+      },
+      'Content');
+
+  t.equal(tag3.toString(),
+    `<div id="banana" class="foo bar" contenteditable ` +
+    `saying="&quot;To light a candle is to cast a shadow...&quot;" ` +
+    `tabindex="413">Content</div>`);
+
+  // 4: attributes match input order
+
+  const tag4 =
+    html.tag('div',
+      {class: ['foo', 'bar'], id: 'banana'},
+      'Content');
+
+  t.equal(tag4.toString(),
+    `<div class="foo bar" id="banana">Content</div>`);
+
+  // 5: multiline contented indented
+
+  const tag5 =
+    html.tag('div', 'foo\nbar');
+
+  t.equal(tag5.toString(),
+    `<div>\n` +
+    `    foo\n` +
+    `    bar\n` +
+    `</div>`);
+
+  // 6: nested multiline content double-indented
+
+  const tag6 =
+    html.tag('div', [
+      html.tag('p',
+        'foo\nbar'),
+      html.tag('span', `I'm on one line!`),
+    ]);
+
+  t.equal(tag6.toString(),
+    `<div>\n` +
+    `    <p>\n` +
+    `        foo\n` +
+    `        bar\n` +
+    `    </p>\n` +
+    `    <span>I'm on one line!</span>\n` +
+    `</div>`);
+
+  // 7: self-closing (with attributes)
+
+  const tag7 =
+    html.tag('article', [
+      html.tag('h1', `Title`),
+      html.tag('hr', {style: `color: magenta`}),
+      html.tag('p', `Shenanigans!`),
+    ]);
+
+  t.equal(tag7.toString(),
+    `<article>\n` +
+    `    <h1>Title</h1>\n` +
+    `    <hr style="color: magenta">\n` +
+    `    <p>Shenanigans!</p>\n` +
+    `</article>`);
+
+  // 8-9: empty tagName passes content through directly
+
+  const tag8 =
+    html.tag(null, [
+      html.tag('h1', `Foo`),
+      html.tag(`h2`, `Bar`),
+    ]);
+
+  t.equal(tag8.toString(),
+    `<h1>Foo</h1>\n` +
+    `<h2>Bar</h2>`);
+
+  const tag9 =
+    html.tag(null, {
+      [html.joinChildren]: html.tag('br'),
+    }, [
+      `Say it with me...`,
+      `Supercalifragilisticexpialidocious!`
+    ]);
+
+  t.equal(tag9.toString(),
+    `Say it with me...\n` +
+    `<br>\n` +
+    `Supercalifragilisticexpialidocious!`);
+});
+
+t.test(`Tag.toString (onlyIfContent)`, t => {
+  t.plan(4);
+
+  // 1-2: basic behavior
+
+  const tag1 =
+    html.tag('div',
+      {[html.onlyIfContent]: true},
+      `Hello!`);
+
+  t.equal(tag1.toString(),
+    `<div>Hello!</div>`);
+
+  const tag2 =
+    html.tag('div',
+      {[html.onlyIfContent]: true},
+      '');
+
+  t.equal(tag2.toString(),
+    '');
+
+  // 3-4: nested onlyIfContent with "more" content
+
+  const tag3 =
+    html.tag('div',
+      {[html.onlyIfContent]: true},
+      [
+        '',
+        0,
+        html.tag('h1',
+          {[html.onlyIfContent]: true},
+          html.tag('strong',
+            {[html.onlyIfContent]: true})),
+        null,
+        false,
+      ]);
+
+  t.equal(tag3.toString(),
+    '');
+
+  const tag4 =
+    html.tag('div',
+      {[html.onlyIfContent]: true},
+      [
+        '',
+        0,
+        html.tag('h1',
+          {[html.onlyIfContent]: true},
+          html.tag('strong')),
+        null,
+        false,
+      ]);
+
+  t.equal(tag4.toString(),
+    `<div><h1><strong></strong></h1></div>`);
+});
+
+t.test(`Tag.toString (joinChildren, noEdgeWhitespace)`, t => {
+  t.plan(6);
+
+  // 1: joinChildren: default (\n), noEdgeWhitespace: true
+
+  const tag1 =
+    html.tag('div',
+      {[html.noEdgeWhitespace]: true},
+      [
+        'Foo',
+        'Bar',
+        'Baz',
+      ]);
+
+  t.equal(tag1.toString(),
+    `<div>Foo\n` +
+    `    Bar\n` +
+    `    Baz</div>`);
+
+  // 2: joinChildren: one-line string, noEdgeWhitespace: default (false)
+
+  const tag2 =
+    html.tag('div',
+      {
+        [html.joinChildren]:
+          html.tag('br', {location: '🍍'}),
+      },
+      [
+        'Foo',
+        'Bar',
+        'Baz',
+      ]);
+
+  t.equal(tag2.toString(),
+    `<div>\n` +
+    `    Foo\n` +
+    `    <br location="🍍">\n` +
+    `    Bar\n` +
+    `    <br location="🍍">\n` +
+    `    Baz\n` +
+    `</div>`);
+
+  // 3-4: joinChildren: blank string, noEdgeWhitespace: default (false)
+
+  const tag3 =
+    html.tag('div',
+      {[html.joinChildren]: ''},
+      [
+        'Foo',
+        'Bar',
+        'Baz',
+      ]);
+
+  t.equal(tag3.toString(),
+    `<div>FooBarBaz</div>`);
+
+  const tag4 =
+    html.tag('div',
+      {[html.joinChildren]: ''},
+      [
+        `Ain't I\na cute one?`,
+        `~`
+      ]);
+
+  t.equal(tag4.toString(),
+    `<div>\n` +
+    `    Ain't I\n` +
+    `    a cute one?~\n` +
+    `</div>`);
+
+  // 5: joinChildren: one-line string, noEdgeWhitespace: true
+
+  const tag5 =
+    html.tag('div',
+      {
+        [html.joinChildren]: html.tag('br'),
+        [html.noEdgeWhitespace]: true,
+      },
+      [
+        'Foo',
+        'Bar',
+        'Baz',
+      ]);
+
+  t.equal(tag5.toString(),
+    `<div>Foo\n` +
+    `    <br>\n` +
+    `    Bar\n` +
+    `    <br>\n` +
+    `    Baz</div>`);
+
+  // 6: joinChildren: empty string, noEdgeWhitespace: true
+
+  const tag6 =
+    html.tag('span',
+      {
+        [html.joinChildren]: '',
+        [html.noEdgeWhitespace]: true,
+      },
+      [
+        html.tag('i', `Oh yes~ `),
+        `You're a cute one`,
+        html.tag('sup', `💕`),
+      ]);
+
+  t.equal(tag6.toString(),
+    `<span><i>Oh yes~ </i>You're a cute one<sup>💕</sup></span>`);
+
+});
+
+t.test(`Tag.toString (custom attributes)`, t => {
+  t.plan(1);
+
+  t.test(`Tag.toString (custom attribute: href)`, t => {
+    t.plan(2);
+
+    const tag1 = html.tag('a', {href: `https://hsmusic.wiki/`});
+    t.equal(tag1.toString(), `<a href="https://hsmusic.wiki/"></a>`);
+
+    const tag2 = html.tag('a', {href: `https://hsmusic.wiki/media/Album Booklet.pdf`});
+    t.equal(tag2.toString(), `<a href="https://hsmusic.wiki/media/Album%20Booklet.pdf"></a>`);
+  });
+});
+
+t.test(`html.template`, t => {
+  t.plan(11);
+
+  let contentCalls;
+
+  // 1-4: basic behavior - no slots
+
+  contentCalls = 0;
+
+  const template1 = html.template({
+    content() {
+      contentCalls++;
+      return html.tag('hr');
+    },
+  });
+
+  t.equal(contentCalls, 0);
+  t.equal(template1.toString(), `<hr>`);
+  t.equal(contentCalls, 1);
+  template1.toString();
+  t.equal(contentCalls, 2);
+
+  // 5-10: basic behavior - slots
+
+  contentCalls = 0;
+
+  const template2 = html.template({
+    slots: {
+      foo: {
+        type: 'string',
+        default: 'Default Message',
+      },
+    },
+
+    content(slots) {
+      contentCalls++;
+      return html.tag('sub', slots.foo.toLowerCase());
+    },
+  });
+
+  t.equal(contentCalls, 0);
+  t.equal(template2.toString(), `<sub>default message</sub>`);
+  t.equal(contentCalls, 1);
+  template2.setSlot('foo', `R-r-really, me?`);
+  t.equal(contentCalls, 1);
+  t.equal(template2.toString(), `<sub>r-r-really, me?</sub>`);
+  t.equal(contentCalls, 2);
+
+  // 11: slot uses default only for null, not falsey
+
+  const template3 = html.template({
+    slots: {
+      slot1: {type: 'number', default: 123},
+      slot2: {type: 'number', default: 456},
+      slot3: {type: 'boolean', default: true},
+      slot4: {type: 'string', default: 'banana'},
+    },
+
+    content(slots) {
+      return html.tag('span', [
+        slots.slot1,
+        slots.slot2,
+        slots.slot3,
+        `(length: ${slots.slot4.length})`,
+      ].join(' '));
+    },
+  });
+
+  template3.setSlots({
+    slot1: null,
+    slot2: 0,
+    slot3: false,
+    slot4: '',
+  });
+
+  t.equal(template3.toString(), `<span>123 0 false (length: 0)</span>`);
+});
+
+t.test(`Template - description errors`, t => {
+  t.plan(14);
+
+  // 1-3: top-level description is object
+
+  strictlyThrows(t,
+    () => Template.validateDescription('snooping as usual'),
+    new TypeError(`Expected object, got string`));
+
+  strictlyThrows(t,
+    () => Template.validateDescription(),
+    new TypeError(`Expected object, got undefined`));
+
+  strictlyThrows(t,
+    () => Template.validateDescription(null),
+    new TypeError(`Expected object, got null`));
+
+  // 4-5: description.content is function
+
+  strictlyThrows(t,
+    () => Template.validateDescription({}),
+    new AggregateError([
+      new TypeError(`Expected description.content`),
+    ], `Errors validating template description`));
+
+  strictlyThrows(t,
+    () => Template.validateDescription({
+      content: 'pingas',
+    }),
+    new AggregateError([
+      new TypeError(`Expected description.content to be function`),
+    ], `Errors validating template description`));
+
+  // 6: aggregate error includes template annotation
+
+  strictlyThrows(t,
+    () => Template.validateDescription({
+      annotation: `my cool template`,
+      content: 'pingas',
+    }),
+    new AggregateError([
+      new TypeError(`Expected description.content to be function`),
+    ], `Errors validating template "my cool template" description`));
+
+  // 7: description.slots is object
+
+  strictlyThrows(t,
+    () => Template.validateDescription({
+      slots: 'pingas',
+      content: () => {},
+    }),
+    new AggregateError([
+      new TypeError(`Expected description.slots to be object`),
+    ], `Errors validating template description`));
+
+  // 8: slot description is object
+
+  strictlyThrows(t,
+    () => Template.validateDescription({
+      slots: {
+        mySlot: 'pingas',
+      },
+
+      content: () => {},
+    }),
+    new AggregateError([
+      new AggregateError([
+        new TypeError(`(mySlot) Expected slot description to be object`),
+      ], `Errors in slot descriptions`),
+    ], `Errors validating template description`))
+
+  // 9-10: slot description has validate or default, not both
+
+  strictlyThrows(t,
+    () => Template.validateDescription({
+      slots: {
+        mySlot: {},
+      },
+      content: () => {},
+    }),
+    new AggregateError([
+      new AggregateError([
+        new TypeError(`(mySlot) Expected either slot validate or type`),
+      ], `Errors in slot descriptions`),
+    ], `Errors validating template description`));
+
+  strictlyThrows(t,
+    () => Template.validateDescription({
+      slots: {
+        mySlot: {
+          validate: 'pingas',
+          type: 'pingas',
+        },
+      },
+      content: () => {},
+    }),
+    new AggregateError([
+      new AggregateError([
+        new TypeError(`(mySlot) Don't specify both slot validate and type`),
+      ], `Errors in slot descriptions`),
+    ], `Errors validating template description`));
+
+  // 11: slot validate is function
+
+  strictlyThrows(t,
+    () => Template.validateDescription({
+      slots: {
+        mySlot: {
+          validate: 'pingas',
+        },
+      },
+      content: () => {},
+    }),
+    new AggregateError([
+      new AggregateError([
+        new TypeError(`(mySlot) Expected slot validate to be function`),
+      ], `Errors in slot descriptions`),
+    ], `Errors validating template description`));
+
+  // 12: slot type is name of built-in type
+
+  strictlyThrows(t,
+    () => Template.validateDescription({
+      slots: {
+        mySlot: {
+          type: 'pingas',
+        },
+      },
+      content: () => {},
+    }),
+    new AggregateError([
+      new AggregateError([
+        /\(mySlot\) Expected slot type to be one of/,
+      ], `Errors in slot descriptions`),
+    ], `Errors validating template description`));
+
+  // 13: slot type has specific errors for function & object
+
+  strictlyThrows(t,
+    () => Template.validateDescription({
+      slots: {
+        slot1: {type: 'function'},
+        slot2: {type: 'object'},
+      },
+      content: () => {},
+    }),
+    new AggregateError([
+      new AggregateError([
+        new TypeError(`(slot1) Functions shouldn't be provided to slots`),
+        new TypeError(`(slot2) Provide validate function instead of type: object`),
+      ], `Errors in slot descriptions`),
+    ], `Errors validating template description`));
+
+  // 14: all intended types are supported
+
+  t.doesNotThrow(
+    () => Template.validateDescription({
+      slots: {
+        slot1: {type: 'string'},
+        slot2: {type: 'number'},
+        slot3: {type: 'bigint'},
+        slot4: {type: 'boolean'},
+        slot5: {type: 'symbol'},
+        slot6: {type: 'html'},
+      },
+      content: () => {},
+    }));
+});
+
+t.test(`Template - slot value errors`, t => {
+  t.plan(8);
+
+  const template1 = html.template({
+    slots: {
+      basicString: {type: 'string'},
+      basicNumber: {type: 'number'},
+      basicBigint: {type: 'bigint'},
+      basicBoolean: {type: 'boolean'},
+      basicSymbol: {type: 'symbol'},
+      basicHTML: {type: 'html'},
+    },
+
+    content: slots =>
+      html.tag('p', [
+        `string: ${slots.basicString}`,
+        `number: ${slots.basicNumber}`,
+        `bigint: ${slots.basicBigint}`,
+        `boolean: ${slots.basicBoolean}`,
+        `symbol: ${slots.basicSymbol?.toString()   ?? 'no symbol'}`,
+
+        `html:`,
+        slots.basicHTML,
+      ]),
+  });
+
+  // 1-2: basic values match type, no error & reflected in content
+
+  t.doesNotThrow(
+    () => template1.setSlots({
+      basicString: 'pingas',
+      basicNumber: 123,
+      basicBigint: 1234567891234567n,
+      basicBoolean: true,
+      basicSymbol: Symbol(`sup`),
+      basicHTML: html.tag('span', `SnooPING AS usual, I see!`),
+    }));
+
+  t.equal(
+    template1.toString(),
+    html.tag('p', [
+      `string: pingas`,
+      `number: 123`,
+      `bigint: 1234567891234567`,
+      `boolean: true`,
+      `symbol: Symbol(sup)`,
+      `html:`,
+      html.tag('span', `SnooPING AS usual, I see!`),
+    ]).toString());
+
+  // 3-4: null matches any type, no error & reflected in content
+
+  t.doesNotThrow(
+    () => template1.setSlots({
+      basicString: null,
+      basicNumber: null,
+      basicBigint: null,
+      basicBoolean: null,
+      basicSymbol: null,
+      basicHTML: null,
+    }));
+
+  t.equal(
+    template1.toString(),
+    html.tag('p', [
+      `string: null`,
+      `number: null`,
+      `bigint: null`,
+      `boolean: null`,
+      `symbol: no symbol`,
+      `html:`,
+    ]).toString());
+
+  // 5-6: type mismatch throws error, invalidates entire setSlots call
+
+  template1.setSlots({
+    basicString: 'pingas',
+    basicNumber: 123,
+  });
+
+  strictlyThrows(t,
+    () => template1.setSlots({
+      basicBoolean: false,
+      basicSymbol: `I'm not a symbol!`,
+    }),
+    new AggregateError([
+      new TypeError(`(basicSymbol) Slot expects symbol, got string`),
+    ], `Error validating template slots`))
+
+  t.equal(
+    template1.toString(),
+    html.tag('p', [
+      `string: pingas`,
+      `number: 123`,
+      `bigint: null`,
+      `boolean: null`,
+      `symbol: no symbol`,
+      `html:`,
+    ]).toString());
+
+  const template2 = html.template({
+    slots: {
+      arrayOfStrings: {
+        validate: v => v.arrayOf(v.isString),
+        default: `Array Of Strings Fallback`.split(' '),
+      },
+
+      arrayOfHTML: {
+        validate: v => v.arrayOf(v.isHTML),
+        default: [],
+      },
+    },
+
+    content: slots =>
+      html.tag('p', [
+        html.tag('strong', slots.arrayOfStrings),
+        `arrayOfHTML length: ${slots.arrayOfHTML.length}`,
+      ]),
+  });
+
+  // 7: isHTML behaves as it should, validate fails with validate throw
+
+  strictlyThrows(t,
+    () => template2.setSlots({
+      arrayOfStrings: ['you got it', 'pingas', 0xdeadbeef],
+      arrayOfHTML: [
+        html.tag('span'),
+        html.template({content: () => 'dog'}),
+        html.blank(),
+        false && 'dogs',
+        null,
+        undefined,
+        html.tags([
+          html.tag('span', 'usual'),
+          html.tag('span', 'i'),
+        ]),
+      ],
+    }),
+    new AggregateError([
+      {
+        name: 'AggregateError',
+        message: /^\(arrayOfStrings\)/,
+        errors: {length: 1},
+      },
+    ], `Error validating template slots`));
+
+  // 8: default slot values respected
+
+  t.equal(
+    template2.toString(),
+    html.tag('p', [
+      html.tag('strong', [
+        `Array`,
+        `Of`,
+        `Strings`,
+        `Fallback`,
+      ]),
+      `arrayOfHTML length: 0`,
+    ]).toString());
+});
+
+t.test(`Stationery`, t => {
+  t.plan(3);
+
+  // 1-3: basic behavior
+
+  const stationery1 = new html.Stationery({
+    slots: {
+      slot1: {type: 'string', default: 'apricot'},
+      slot2: {type: 'string', default: 'disaster'},
+    },
+
+    content: ({slot1, slot2}) => html.tag('span', `${slot1} ${slot2}`),
+  });
+
+  const template1 = stationery1.template();
+  const template2 = stationery1.template();
+
+  template2.setSlots({slot1: 'aquaduct', slot2: 'dichotomy'});
+
+  const template3 = stationery1.template();
+
+  template3.setSlots({slot2: 'vinaigrette'});
+
+  t.equal(template1.toString(), `<span>apricot disaster</span>`);
+  t.equal(template2.toString(), `<span>aquaduct dichotomy</span>`);
+  t.equal(template3.toString(), `<span>apricot vinaigrette</span>`);
+});