diff --git a/spec/content_css_sanitize_spec.lua b/spec/content_css_sanitize_spec.lua
new file mode 100644
index 0000000..e068804
--- /dev/null
+++ b/spec/content_css_sanitize_spec.lua
@@ -0,0 +1,139 @@
+package.path = "./?.lua;./?/init.lua;" .. package.path
+
+local checks = 0
+local function expect(condition, message)
+    checks = checks + 1
+    if not condition then error(message or ("check " .. checks .. " failed")) end
+end
+
+package.preload["logger"] = function()
+    return {
+        info = function() end,
+        warn = function() end,
+        err = function() end,
+        dbg = function() end,
+    }
+end
+package.preload["weread.lib.crypto"] = function() return {} end
+package.preload["weread.lib.reader_state"] = function() return {} end
+package.preload["weread.lib.protocol"] = function() return {} end
+package.preload["weread.lib.thoughts"] = function() return {} end
+
+local Content = require("weread/lib/content")
+
+-- The hostile rule observed in real WeRead e_2 shards: crengine honors the
+-- root-element zero sizing and p{font-size:1rem} then collapses the whole book.
+local hostile = "html,\nbody {\n  margin: 0;\n  padding: 0;\n  font-size: 0;\n  }"
+local cleaned, count = Content.sanitize_book_css(hostile)
+expect(count == 1, "the single font-size:0 declaration must be reported as one removal")
+expect(not cleaned:find("font%-size"), "font-size:0 declaration survived sanitization")
+expect(cleaned:find("{", 1, true) and cleaned:find("}", 1, true),
+    "sanitization must keep the rule braces intact")
+expect(cleaned:find("margin: 0;", 1, true) and cleaned:find("padding: 0;", 1, true),
+    "sibling margin/padding declarations must survive sanitization")
+
+-- Review: `font-size: 0` can be intentional outside the root elements (e.g.
+-- hiding whitespace between inline-block items), so only rules whose selector
+-- list names exactly html/body may be touched.
+cleaned, count = Content.sanitize_book_css(".a{font-size: 0 !important;color:red}")
+expect(count == 0 and cleaned == ".a{font-size: 0 !important;color:red}",
+    "a non-root selector had its intentional font-size:0 removed")
+
+cleaned, count = Content.sanitize_book_css(".slide-nav { font-size: 0 }")
+expect(count == 0 and cleaned == ".slide-nav { font-size: 0 }",
+    "an inline-block whitespace trick on a class selector was stripped")
+
+cleaned, count = Content.sanitize_book_css("body p{font-size:0}")
+expect(count == 0 and cleaned == "body p{font-size:0}",
+    "a descendant-of-body selector was treated as the root element")
+
+cleaned, count = Content.sanitize_book_css("body,.wrapper{font-size:0}")
+expect(count == 0 and cleaned == "body,.wrapper{font-size:0}",
+    "a selector list mixing body with other targets was treated as root-only")
+
+cleaned, count = Content.sanitize_book_css("HTML ,\nBODY { font-size: 0 }")
+expect(count == 1 and not cleaned:find("font%-size"),
+    "root selector matching must be case- and whitespace-insensitive")
+
+-- Deliberate limits: :root and @media-wrapped root rules stay untouched.
+cleaned, count = Content.sanitize_book_css(":root{font-size:0}")
+expect(count == 0 and cleaned == ":root{font-size:0}",
+    ":root sizing was stripped although only html/body rules are covered")
+
+local media_css = "@media print { html, body { font-size: 0 } }"
+cleaned, count = Content.sanitize_book_css(media_css)
+expect(count == 0 and cleaned == media_css,
+    "@media-wrapped root sizing was stripped although only top-level rules are covered")
+
+-- An at-rule before the selector must not prevent matching the rule itself.
+cleaned, count = Content.sanitize_book_css('@charset "utf-8";html,body{font-size:0}')
+expect(count == 1 and not cleaned:find("font%-size"),
+    "a preceding at-rule broke root-selector matching")
+
+cleaned, count = Content.sanitize_book_css("html{font-size: 0px}body{font-size: 0%;}")
+expect(count == 2 and not cleaned:find("font%-size"),
+    "zero font-size with px/% units was not fully removed on root selectors")
+
+cleaned, count = Content.sanitize_book_css(".fs05 { font-size: 0.5rem; }")
+expect(count == 0 and cleaned == ".fs05 { font-size: 0.5rem; }",
+    "fractional font-size must stay untouched")
+
+cleaned, count = Content.sanitize_book_css("html, body { font-size: 0.5rem; }")
+expect(count == 0 and cleaned == "html, body { font-size: 0.5rem; }",
+    "fractional root font-size must stay untouched")
+
+cleaned, count = Content.sanitize_book_css("p { font-size: 1rem; }")
+expect(count == 0 and cleaned == "p { font-size: 1rem; }",
+    "non-zero font-size must stay untouched")
+
+cleaned, count = Content.sanitize_book_css("{ color: #000; font-size: 0\n}")
+expect(count == 0 and cleaned:find("font-size: 0", 1, true),
+    "a block without a selector must be left untouched")
+
+local passthrough, passthrough_count = Content.sanitize_book_css(nil)
+expect(passthrough == nil and passthrough_count == 0,
+    "nil input must pass through unchanged with count 0")
+passthrough, passthrough_count = Content.sanitize_book_css("")
+expect(passthrough == "" and passthrough_count == 0,
+    "empty input must pass through unchanged with count 0")
+
+-- Integration-style: sanitizing an already-sanitized shard changes nothing.
+-- Only the root rule qualifies; the other blocks keep their declarations.
+local combined = table.concat({
+    hostile,
+    ".a{font-size: 0 !important;color:red}",
+    "p{font-size: 0px}h1{font-size: 0%;}",
+    "{ color: #000; font-size: 0\n}",
+}, "\n")
+local once, first_count = Content.sanitize_book_css(combined)
+local twice, second_count = Content.sanitize_book_css(once)
+expect(first_count == 1, "combined shard should lose only the root font-size declaration")
+expect(second_count == 0 and twice == once, "sanitization must be idempotent")
+
+-- Adjacent zero declarations: each pass consumes one boundary character, so
+-- the sanitizer must iterate to a fixpoint instead of keeping the second one.
+cleaned, count = Content.sanitize_book_css("html,body{font-size:0;font-size:0}")
+expect(count == 2, "adjacent zero declarations must both be removed")
+expect(not cleaned:find("font%-size"), "adjacent removal left a hostile declaration behind")
+expect(cleaned:find("^html,body%{.*%}$"), "block structure was damaged by adjacent removal")
+
+cleaned, count = Content.sanitize_book_css("html,body{font-size:0 ;font-size:0 ;color:red}")
+expect(count == 2 and cleaned:find("color:red", 1, true),
+    "spaced adjacent zeros must be removed while siblings survive")
+
+cleaned, count = Content.sanitize_book_css("html,body{font-size:0vh}")
+expect(count == 1 and not cleaned:find("font%-size"),
+    "zero with any letter unit (0vh) is a zero length and must be removed")
+
+-- A CSS comment carrying the exact declaration may lose its interior, but the
+-- stylesheet structure around it must survive and the result stays idempotent.
+local commented = "body{ /* font-size: 0 ; old */ color:blue }"
+cleaned, count = Content.sanitize_book_css(commented)
+local open_braces = select(2, cleaned:gsub("%{", ""))
+local close_braces = select(2, cleaned:gsub("}", ""))
+expect(count == 1 and cleaned:find("color:blue", 1, true) and open_braces == close_braces,
+    "comment rewrite must keep the block structurally valid")
+local _, recount = Content.sanitize_book_css(cleaned)
+expect(recount == 0, "sanitization after comment rewrite must be idempotent")
+
+print(("content_css_sanitize_spec: %d checks"):format(checks))
diff --git a/weread/lib/content.lua b/weread/lib/content.lua
index 9f27f16..4a1cf6e 100755
--- a/weread/lib/content.lua
+++ b/weread/lib/content.lua
@@ -1362,12 +1362,108 @@ function Content.fetch_chapter_xhtml(client, settings, book, chapter)
     )
 end
 
+-- True when the text following the literal "0" of a font-size declaration
+-- (already captured by the caller's pattern) is only an optional unit plus
+-- whitespace and an optional !important flag, i.e. the declared size really is
+-- zero. Zero times any unit is still zero length, so an empty tail (bare 0) and
+-- every letter-unit form (px/em/rem/vh/...) count; fractional sizes such as
+-- 0.5rem never match because "." is not a letter.
+local function is_zero_font_size(tail)
+    local value = tail:lower():match("^%s*(.-)%s*$")
+    if value:sub(-10) == "!important" then
+        value = (value:match("^(.-)%s*!important$") or ""):match("^%s*(.-)%s*$")
+    end
+    return value == "" or value == "%" or value:match("^%a+$") ~= nil
+end
+
+-- Known limitations: property-name matching is case-sensitive (all observed
+-- WeRead shards are lowercase), a CSS comment containing exactly
+-- "font-size: 0" may have its interior rewritten without structural harm, and
+-- only top-level rules naming exactly html/body are touched (:root,
+-- descendant selectors and @media-wrapped rules are left as-is).
+
+-- True when the selector list names nothing but the root elements, i.e. every
+-- comma-separated selector is exactly html or body (case- and
+-- whitespace-insensitive). Compound selectors such as "body p" or
+-- "body, .wrapper" also style other content, so they never qualify.
+local function is_root_selector_list(selectors)
+    local count = 0
+    for selector in (selectors or ""):gmatch("[^,]+") do
+        count = count + 1
+        local name = selector:lower():gsub("^%s+", ""):gsub("%s+$", "")
+        if name ~= "html" and name ~= "body" then return false end
+    end
+    return count > 0
+end
+
+-- Remove every zero `font-size` declaration from one braceless declaration
+-- block. The sentinel "{" guarantees the boundary capture below always has a
+-- character to inspect, even when the declaration opens the block.
+local function strip_zero_font_sizes(block)
+    local removed = 0
+    local cleaned = ("{" .. block):gsub("([^%w%-])(%s*)font%-size%s*:%s*0([^;}]*)(;?)", function(boundary, leading, tail, _terminator)
+        if not is_zero_font_size(tail) then
+            return nil -- keep fractional sizes such as 0.5rem untouched
+        end
+        removed = removed + 1
+        return boundary .. leading
+    end)
+    return cleaned:sub(2), removed
+end
+
+-- Strip hostile `font-size: 0` declarations from server-provided book css, but
+-- only inside rules whose selector list is exactly `html` and/or `body`.
+-- WeRead shards occasionally ship `html, body { ... font-size: 0; }`; WeRead's
+-- own apps ignore root-element sizing but crengine honors it, collapsing the
+-- whole book to a near-zero font size on device. Elsewhere `font-size: 0` can
+-- be intentional (e.g. hiding whitespace between inline-block items), so every
+-- other rule passes through verbatim.
+local function sanitize_book_css_pass(css)
+    local removed = 0
+    -- Scan whole `selector { block }` units (balanced braces); untouched units
+    -- are returned verbatim so no other declaration can be disturbed.
+    local sanitized = css:gsub("([^{}]*)(%b{})", function(prelude, block)
+        -- Text before the last ";" belongs to an at-rule or a previous
+        -- statement, not to this block's selector list.
+        local selectors = prelude:match("[^;]*$") or ""
+        if not is_root_selector_list(selectors) then
+            return prelude .. block
+        end
+        local cleaned, dropped = strip_zero_font_sizes(block:sub(2, -2))
+        removed = removed + dropped
+        return prelude .. "{" .. cleaned .. "}"
+    end)
+    return sanitized, removed
+end
+
+function Content.sanitize_book_css(css)
+    if type(css) ~= "string" or css == "" then
+        return css, 0
+    end
+    -- Each pass consumes one boundary character per match, so adjacent zero
+    -- declarations ("font-size:0;font-size:0") need repeated passes until the
+    -- fixpoint; the cap only guards pathological input.
+    local removed_total = 0
+    local sanitized = css
+    for _i = 1, 16 do
+        local removed
+        sanitized, removed = sanitize_book_css_pass(sanitized)
+        removed_total = removed_total + removed
+        if removed == 0 then break end
+    end
+    return sanitized, removed_total
+end
+
 function Content.fetch_chapter_css(client, settings, book, chapter)
     local ok, css = pcall(function()
         return Content.decode_content_shard(Content.fetch_chapter_shard(client, settings, book, chapter, "/web/book/chapter/e_2"))
     end)
     if ok then
-        return css
+        local sanitized, removed = Content.sanitize_book_css(css)
+        if removed > 0 then
+            logger.warn("removed ", removed, " hostile font-size:0 declarations from book css")
+        end
+        return sanitized
     end
     return nil
 end