An attribute must have a value (#36450)

* Attribute can't have no value

HTML attributes by definition are name-value pairs and can't have no value. See [this article](https://8hob.io/posts/set-attribute-without-value-in-js/#an-attribute-always-has-a-value) for a quick explanation.

Relevant HTML standard: https://html.spec.whatwg.org/multipage/syntax.html#attributes-2

* Use m-dash instead of hyphen

* Clarify implicit attribute value

* consistent commas

* Update files/en-us/glossary/boolean/html/index.md

* Grammar changes

* Update files/en-us/glossary/boolean/html/index.md

* Consistent semicolons

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Co-authored-by: sideshowbarker <[email protected]>

files/en-us/glossary/boolean/html/index.md

@@ -6,13 +6,13 @@ page-type: glossary-definition
 
 {{GlossarySidebar}}
 
-A **boolean attribute** in {{Glossary("HTML")}} is an {{glossary("attribute")}} that represents `true` or `false` values. If an HTML tag contains a boolean attribute - no matter the value of that attribute - the attribute is set to `true` on that element. If an HTML tag does not contain the attribute, the attribute is set to `false`.
+A **boolean attribute** in {{Glossary("HTML")}} is an {{glossary("attribute")}} that represents `true` or `false` values. If an HTML tag contains a boolean attribute — no matter the value of that attribute — the attribute is set to `true` on that element. If an HTML tag does not contain the attribute, the attribute is set to `false`.
 
-If the attribute is present, it can have one of the following values:
+If the attribute is present, it can have one of the following forms:
 
-- no value at all, e.g. `attribute`
-- the empty string, e.g. `attribute=""`
-- attribute's name itself, with no leading or trailing whitespace, e.g. `attribute="attribute"`
+- the attribute name alone; e.g., `attribute`, meaning its implicit value is the empty string
+- the attribute with a value of the empty string; e.g., `attribute=""`
+- the attribute with a value of the attribute's name itself, with no leading or trailing whitespace; e.g., `attribute="attribute"`
 
 > [!NOTE]
 > The strings "true" and "false" are invalid values. To set the attribute to `false`, the attribute should be omitted altogether. Though modern browsers treat _any_ string value as `true`, you should not rely on that behavior.