Compatibility Matrix v1
Legend: V verified, P partially verified, D primary documentation only, C current community/competitor report, U unknown, N/A not applicable.
Every V cell must link to a dated test record and screenshot before publication. V means the behavior was verified, not that the construct passed unchanged.
| Case | Construct | Substack | Beehiiv | X | WordPress | Kit | Ghost | Current note | |
|---|---|---|---|---|---|---|---|---|---|
| T01 | Inline formatting and links | V | V | V | V | V | U | D | WordPress converted ordinary inline Markdown to native Gutenberg markup; relative and anchor links still require validation |
| T02 | H1–H6 hierarchy | V | V | V | P | V | U | D | WordPress preserved H1–H6, but the document H1 did not populate the separate post title |
| T03 | Nested lists and tasks | V | V | V | P | V | U | D | WordPress preserved nested lists but lost task state in Gutenberg paste; Markdown block displayed literal task markers |
| T04 | Quote and divider | V | V | V | P | V | U | D | WordPress converted quote and divider syntax into native blocks |
| T05 | Fenced and long-line code | V | V | V | P | V | U | D | WordPress created code blocks and detected JavaScript; tested long lines could overflow narrow themes |
| T06 | Small GFM table | V | V | V | V | V | U | U | WordPress Gutenberg and Markdown-block modes both rendered the tested GFM table |
| T07 | Wide table and mobile/email | V | V | V | U | P | U | U | WordPress created the wide table, but reliable visual mobile evidence remains theme-sensitive and incomplete |
| T08 | Remote and local images | V | V | V | P | V | U | D | WordPress hotlinked the remote image with alt/title; the local relative image was broken and needs upload |
| T09 | Markdown footnotes | V | V | V | U | V | U | D | Gutenberg paste left footnotes literal; WordPress.com Markdown block rendered references, notes and backlinks |
| T10 | Inline/block math | V | V | V | U | V | U | U | Both tested WordPress modes left dollar-delimited math literal; native Math conversion remains untested |
| T11 | Mermaid fence | V | V | V | U | V | U | U | WordPress preserved Mermaid source as code but did not render a diagram |
| T12 | Obsidian wiki links/callouts/transclusions | V | V | V | U | V | U | U | WordPress left Obsidian links, transclusion and callout markers literal |
| T13 | Raw HTML/details/mark | V | V | V | U | V | U | U | Gutenberg flattened Details structure; Markdown block escaped raw HTML |
| T14 | Hard breaks and paragraphs | V | V | V | V | V | U | U | WordPress preserved the tested hard break and paragraph separation |
| T15 | Escaping and literal characters | V | V | V | V | V | U | U | WordPress displayed escaped markers literally and encoded unsafe literal symbols in serialized output |
| T16 | Heading anchor link | V | V | V | U | V | U | U | WordPress retained the anchor href but created no matching heading ID; the link was broken |
Documented platform-level facts
| Platform | Claim | Evidence | Level |
|---|---|---|---|
| Substack | Full Markdown documents are not supported in the post editor | https://support.substack.com/hc/en-us/articles/360037463132-Do-you-support-Markdown | D |
| Substack | H1–H3, lists, quote, divider, emphasis and link typing shortcuts exist | https://support.substack.com/hc/en-us/articles/5582336874132-Can-I-use-shortcuts-in-the-Substack-editor | D |
| Substack | Native LaTeX blocks and inline footnotes exist | https://on.substack.com/p/product-news-jan-23 | D |
| Beehiiv | Visual Editor documents H1–H6, tables, footnotes, code blocks and HTML snippets | https://www.beehiiv.com/support/article/31326914767639-options-on-the-compose-page-of-the-post-flow | D |
| X | Standard posts use a 280 weighted-character limit; URLs weigh 23 and emoji/CJK normally weigh 2 | https://docs.x.com/fundamentals/counting-characters | D |
| X | Premium longer posts can contain up to 25,000 characters; web draft/scheduling limitations apply | https://help.x.com/en/using-x/types-of-posts | D |
| Standard posts allow up to 3,000 characters; longer material can use the separate article workflow | https://www.linkedin.com/help/linkedin/answer/a528176 | D | |
| Pasting or typing a URL may generate a link preview | https://www.linkedin.com/help/linkedin/answer/a525301 | D | |
| The Posts API supports text and several separate media/structured content types | https://learn.microsoft.com/en-us/linkedin/marketing/community-management/shares/posts-api?view=li-lms-2026-04 | D | |
| WordPress.com | The Markdown block follows CommonMark, requires Markdown to be enabled and retains source Markdown for later editing | https://wordpress.com/support/wordpress-editor/blocks/markdown-block/ | D |
| WordPress | The Posts REST endpoint creates drafts/published posts and accepts a content field |
https://developer.wordpress.org/rest-api/reference/posts/ | D |
| WordPress | Media uploads use a separate REST endpoint with alt-text, caption and description fields | https://developer.wordpress.org/rest-api/reference/media/ | D |
| WordPress | Core native footnotes were introduced in WordPress 6.3 | https://wordpress.org/documentation/article/footnotes-block/ | D |
| Ghost | Ghost parses Markdown and supports a Markdown card with footnotes | https://ghost.org/help/using-markdown/ | D |
Verified Substack mode summary
| Mode | Outcome | Test record |
|---|---|---|
| Raw Markdown paste | Formatting failed across the fixture; syntax stayed literal | 2026-08-24 record |
| Marked 16.2.1 rich paste | Ordinary structure mostly survived; tables, footnotes, math, Mermaid, local images and Obsidian constructs required repair | 2026-08-24 record |
| Native Footnote and LaTeX | Both rendered in saved mobile preview | 2026-08-24 record |
Verified Beehiiv mode summary
| Mode | Outcome | Test record |
|---|---|---|
| Raw Markdown paste | Preferred: Beehiiv parsed ordinary Markdown and GFM tables into native blocks; fragile constructs still require repairs | 2026-08-24 record |
| Marked 16.2.1 rich paste | Ordinary structure survived, but front matter and task state regressed relative to raw paste | 2026-08-24 record |
| Native Footnotes and HTML snippet | Both rendered in unpublished email and web previews; no visible math control | 2026-08-24 record |
Verified X mode summary
| Mode | Outcome | Test record |
|---|---|---|
| Raw Markdown | Markdown syntax remained literal; line breaks survived and the URL substring was detected | 2026-08-24 record |
| Cleaned plain text | Preferred: readable paragraphs, bullets, visible URL and labels without Markdown residue | 2026-08-24 record |
| Standard post count | 280 ASCII accepted; 281 rejected; long URL weighted as 23; emoji weighted as 2 | 2026-08-24 record |
| Native thread composer | Add post created a second editor and Post all action; nothing was submitted |
2026-08-24 record |
Verified LinkedIn mode summary
| Mode | Outcome | Test record |
|---|---|---|
| Raw Markdown | Markdown syntax remained literal; Markdown links did not generate a preview | 2026-08-24 record |
| Cleaned plain text | Preferred: paragraphs and bullets stayed readable; the bare URL generated LinkedIn's preview card | 2026-08-24 record |
| Standard post count | 3,000 ASCII accepted; 3,001 rejected; full URL length counted; tested emoji consumed two UTF-16 units | 2026-08-24 record |
| Separate article workflow | Visible from the feed and documented for longer content; its editor was not tested in this pass | 2026-08-24 record |
Verified WordPress mode summary
| Mode | Outcome | Test record |
|---|---|---|
| Gutenberg raw Markdown paste | Strong preferred clipboard mode: ordinary Markdown became editable native blocks; front matter, title, tasks, local media and specialized syntax need repairs | 2026-08-24 record |
| Saved Gutenberg preview | Confirmed semantic structure, remote-image hotlinking, broken local media, literal footnotes/math/Obsidian and missing heading IDs | 2026-08-24 record |
| WordPress.com/Jetpack Markdown block | Source-preserving optional mode: rendered tables and footnotes but not math, Mermaid diagrams, raw HTML or Obsidian syntax | 2026-08-24 record |
| REST publisher | Official Posts and Media endpoints support a future authenticated draft-first flow; not live-tested | 2026-08-24 record |