diff --git a/README.md b/README.md
index dcdf689..667c6fa 100644
--- a/README.md
+++ b/README.md
@@ -55,11 +55,8 @@ require('preview').setup({
 **Q: How do I override a preset?**
 
 ```lua
-local presets = require('preview.presets')
 require('preview').setup({
-  typst = vim.tbl_deep_extend('force', presets.typst, {
-    env = { TYPST_FONT_PATHS = '/usr/share/fonts' },
-  }),
+  typst = { env = { TYPST_FONT_PATHS = '/usr/share/fonts' } },
 })
 ```
 
@@ -70,8 +67,8 @@ open the output with `vim.ui.open()` after the first successful compilation.
 For a specific application, pass a command table:
 
 ```lua
-typst = vim.tbl_deep_extend('force', presets.typst, {
-  open = { 'sioyek', '--new-instance' },
+require('preview').setup({
+  typst = { open = { 'sioyek', '--new-instance' } },
 })
 ```
 
diff --git a/doc/preview.nvim.txt b/doc/preview.nvim.txt
index 3ed587a..753fefc 100644
--- a/doc/preview.nvim.txt
+++ b/doc/preview.nvim.txt
@@ -42,9 +42,18 @@ Configure via `require('preview').setup()`.
                                                             *preview.setup()*
 setup({opts?})
 
-  `opts` is a mixed table. Array entries are preset names (see
-  |preview.nvim-presets|). Hash entries with table values are custom
-  provider configs keyed by filetype.
+  `opts` is a table where keys are preset names or filetypes. For each
+  key `k` with value `v` (excluding `debug`):
+
+  - If `k` is a preset name and `v` is `true`, the preset is registered
+    as-is under its filetype.
+  - If `k` is a preset name and `v` is a table, it is deep-merged with
+    the preset and registered under the preset's filetype.
+  - If `k` is not a preset name and `v` is a table, it is registered
+    directly as a custom provider keyed by filetype `k`.
+  - If `v` is `false`, the entry is skipped (no-op).
+
+  See |preview.nvim-presets| for available preset names.
 
   Fields:~
 
@@ -91,33 +100,28 @@ Context fields:~
   `root`    string     Project root (git root or file directory).
   `ft`      string     Filetype.
 
-Example using preset names:~
+Example enabling presets:~
 >lua
-    require('preview').setup({ 'typst', 'latex', 'markdown' })
+    require('preview').setup({ typst = true, latex = true, github = true })
 <
 
-Example with a custom provider:~
+Example overriding a preset field:~
 >lua
     require('preview').setup({
-      typst = {
-        cmd = { 'typst', 'compile' },
+      typst = { open = { 'sioyek', '--new-instance' } },
+    })
+<
+
+Example with a fully custom provider (key is not a preset name):~
+>lua
+    require('preview').setup({
+      rst = {
+        cmd = { 'rst2html' },
         args = function(ctx)
           return { ctx.file }
         end,
         output = function(ctx)
-          return ctx.file:gsub('%.typ$', '.pdf')
-        end,
-        error_parser = function(stderr, ctx)
-          local diagnostics = {}
-          for line, col, msg in stderr:gmatch('error:.-(%d+):(%d+):%s*(.-)%\n') do
-            table.insert(diagnostics, {
-              lnum = tonumber(line) - 1,
-              col = tonumber(col) - 1,
-              message = msg,
-              severity = vim.diagnostic.severity.ERROR,
-            })
-          end
-          return diagnostics
+          return ctx.file:gsub('%.rst$', '.html')
         end,
       },
     })
@@ -132,20 +136,17 @@ Import them from `preview.presets`:
   `presets.typst`              typst compile → PDF
   `presets.latex`              latexmk -pdf → PDF (with clean support)
   `presets.markdown`           pandoc → HTML (standalone, embedded)
-  `presets.github`             pandoc → HTML (GitHub-styled)
+  `presets.github`             pandoc → HTML (GitHub-styled, `-f gfm` input)
 
-Pass preset names as array entries to `setup()`:
+Enable presets with `preset_name = true`:
 >lua
-    require('preview').setup({ 'typst', 'latex', 'markdown' })
+    require('preview').setup({ typst = true, latex = true, github = true })
 <
 
-Override individual fields using `vim.tbl_deep_extend`:
+Override individual fields by passing a table instead of `true`:
 >lua
-    local presets = require('preview.presets')
     require('preview').setup({
-      typst = vim.tbl_deep_extend('force', presets.typst, {
-        env = { TYPST_FONT_PATHS = '/usr/share/fonts' },
-      }),
+      typst = { env = { TYPST_FONT_PATHS = '/usr/share/fonts' } },
     })
 <
 
diff --git a/lua/preview/init.lua b/lua/preview/init.lua
index 968b201..e2cf794 100644
--- a/lua/preview/init.lua
+++ b/lua/preview/init.lua
@@ -68,15 +68,17 @@ function M.setup(opts)
     if k == 'debug' then
       vim.validate('preview.setup opts.debug', v, { 'boolean', 'string' })
       debug = v
-    elseif type(k) == 'number' then
-      vim.validate('preview.setup preset name', v, 'string')
-      local preset = presets[v]
+    elseif type(k) ~= 'number' then
+      local preset = presets[k]
       if preset then
-        providers[preset.ft] = preset
+        if v == true then
+          providers[preset.ft] = preset
+        elseif type(v) == 'table' then
+          providers[preset.ft] = vim.tbl_deep_extend('force', preset, v)
+        end
+      elseif type(v) == 'table' then
+        providers[k] = v
       end
-    else
-      vim.validate('preview.setup provider config', v, 'table')
-      providers[k] = v
     end
   end
 
diff --git a/lua/preview/presets.lua b/lua/preview/presets.lua
index 3579a8d..098b939 100644
--- a/lua/preview/presets.lua
+++ b/lua/preview/presets.lua
@@ -53,11 +53,13 @@ M.github = {
   args = function(ctx)
     local output = ctx.file:gsub('%.md$', '.html')
     return {
+      '-f',
+      'gfm',
       ctx.file,
       '-s',
       '--embed-resources',
       '--css',
-      'https://cdn.jsdelivr.net/gh/pixelbrackets/gfm-stylesheet@master/github.css',
+      'https://cdn.jsdelivr.net/gh/pixelbrackets/gfm-stylesheet@master/dist/gfm.css',
       '-o',
       output,
     }
diff --git a/spec/init_spec.lua b/spec/init_spec.lua
index 7921aeb..5b97c8a 100644
--- a/spec/init_spec.lua
+++ b/spec/init_spec.lua
@@ -22,7 +22,7 @@ describe('preview', function()
       assert.are.same({}, config.providers)
     end)
 
-    it('accepts full provider config via hash entry', function()
+    it('merges override table with matching preset', function()
       helpers.reset_config({
         typst = {
           cmd = { 'typst', 'compile' },
@@ -33,8 +33,8 @@ describe('preview', function()
       assert.is_not_nil(config.providers.typst)
     end)
 
-    it('resolves array preset names to provider configs', function()
-      helpers.reset_config({ 'typst', 'markdown' })
+    it('resolves preset = true to provider config', function()
+      helpers.reset_config({ typst = true, markdown = true })
       local config = require('preview').get_config()
       local presets = require('preview.presets')
       assert.are.same(presets.typst, config.providers.typst)
@@ -42,14 +42,14 @@ describe('preview', function()
     end)
 
     it('resolves latex preset under tex filetype', function()
-      helpers.reset_config({ 'latex' })
+      helpers.reset_config({ latex = true })
       local config = require('preview').get_config()
       local presets = require('preview.presets')
       assert.are.same(presets.latex, config.providers.tex)
     end)
 
     it('resolves github preset under markdown filetype', function()
-      helpers.reset_config({ 'github' })
+      helpers.reset_config({ github = true })
       local config = require('preview').get_config()
       local presets = require('preview.presets')
       assert.are.same(presets.github, config.providers.markdown)
@@ -59,7 +59,7 @@ describe('preview', function()
   describe('resolve_provider', function()
     before_each(function()
       helpers.reset_config({
-        typst = { cmd = { 'typst', 'compile' } },
+        typst = true,
       })
       preview = require('preview')
     end)
diff --git a/spec/presets_spec.lua b/spec/presets_spec.lua
index 2eba98f..b8572c5 100644
--- a/spec/presets_spec.lua
+++ b/spec/presets_spec.lua
@@ -139,16 +139,31 @@ describe('presets', function()
       local args = presets.github.args(md_ctx)
       assert.is_table(args)
       assert.are.same({
+        '-f',
+        'gfm',
         '/tmp/document.md',
         '-s',
         '--embed-resources',
         '--css',
-        'https://cdn.jsdelivr.net/gh/pixelbrackets/gfm-stylesheet@master/github.css',
+        'https://cdn.jsdelivr.net/gh/pixelbrackets/gfm-stylesheet@master/dist/gfm.css',
         '-o',
         '/tmp/document.html',
       }, args)
     end)
 
+    it('args include -f and gfm flags', function()
+      local args = presets.github.args(md_ctx)
+      local idx = nil
+      for i, v in ipairs(args) do
+        if v == '-f' then
+          idx = i
+          break
+        end
+      end
+      assert.is_not_nil(idx)
+      assert.are.equal('gfm', args[idx + 1])
+    end)
+
     it('returns html output path', function()
       local output = presets.github.output(md_ctx)
       assert.is_string(output)