Add README.md to dom/html-related directories in LayoutTests.

third_party/WebKit/LayoutTests/html/README.md

Index: third_party/WebKit/LayoutTests/html/README.md
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+# html
+
+This directory contains tests related to [HTML standard](https://html.spec.whatwg.org/).
+
+Please do not add tests unrelated to HTML standard.
+
+## Exceptions
+
+Some areas have their own directories not under html/.
+
+*   Canvas: ../fast/canvas/ or ../canvas/
+*   Editing: ../editing/
+*   Forms: [../fast/forms/](../fast/forms/README.md)
+*   History API: ../fast/history/
+*   Media elements: ../media/
+*   Navigation/loading: ../fast/loader/, ../loader/, or ../http/tests/loading/
+
+
+## Sub-directories
+
+*   details_summary/
+
+    DETAILS and SUMMARY elements.
+
+*   dialog/
+
+    DIALOG element.
+
+*   document-metadata/
+
+    [Document metadata elements](https://html.spec.whatwg.org/multipage/semantics.html#document-metadata);
+    HEAD, TITLE, BASE, LINK, META, and STYLE.
+
+*   grouping-content/
+
+    [Grouping content elmeents](https://html.spec.whatwg.org/multipage/semantics.html#grouping-content);
+    P, HR, PRE, BLOCKQUOTE, OL, UL, LI, DL, DT, DD, FIGURE, FIGCAPTION, MAIN,
+    and DIV.
+
+*   marquee/
+
+    MARQUEE element.
+
+*   sections/
+
+    [Sections elements](https://html.spec.whatwg.org/multipage/semantics.html#sections);
+    BODY, ARTICLE, SECTION, NAV, ASIDE, Hn, HGROUP, HEADER, FOOTER, and ADDRESS.
+
+*   text-level-semantics/
+
+    [Text-level semantics elements](https://html.spec.whatwg.org/multipage/semantics.html#text-level-semantics);
+    A, EM, STRONG, SMALL, S, CITE, Q, DFN, ABBR, RUBY, RT, RP, DATA, TIME, CODE,
+    VAR, SAMP, KBD, SUB, SUP, I, B, U, MARK, BDI, BDO, SPAN, BR, and WBR.