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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchUse an in-memory reader when the Go API expects io.Reader: wrap your CSS with strings.NewReader(cssText) or bytes.NewBufferString(cssText), then pass it to the parser. If the library accepts strings directly, pass the string without an adapter. Loading text for parsing is not the same as applying CSS in a browser or fetching linked stylesheets.
Choose the input shape your CSS library expects
Go strings are already in memory, so a temporary file is unnecessary for a reader-based parser. The standard-library adapters cover the two common cases:
strings.NewReader(cssText)returns anio.Readerover a string.bytes.NewBufferString(cssText)returns a byte buffer that also satisfiesio.Reader.
The tdewolff/parse/v2/css package documents a reader-oriented parser. The aymerick/douceur parser documents a direct string interface. Pick according to the output you need: token or grammar iteration versus a parsed stylesheet representation.
Parse a stylesheet string with tdewolff/parse
This example follows the v2 parser shape documented by the package. It treats the input as a complete stylesheet, not the contents of a style attribute.
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package main
import (
"fmt"
"io"
"strings"
"github.com/tdewolff/parse/v2"
"github.com/tdewolff/parse/v2/css"
)
func main() {
cssText := `body { color: rebeccapurple; }`
input := parse.NewInput(strings.NewReader(cssText))
p := css.NewParser(input, false) // false means a stylesheet, not inline declarations
for {
grammar, _, data := p.Next()
if grammar == css.ErrorGrammar {
break
}
// Inspect grammar/data, or call p.Values() when token values are needed.
fmt.Printf("grammar=%v data=%qn", grammar, data)
}
if err := p.Err(); err != nil && err != io.EOF {
panic(err)
}
}The important operation is the conversion from the string to a reader. The parser’s Next() method advances through grammar units. Stop on css.ErrorGrammar, then inspect Err(); do not assume that every stop means successful parsing. Check the exact version you select because APIs can change between dependency versions.
Use bytes.NewBufferString instead
input := parse.NewInput(bytes.NewBufferString(cssText))
p := css.NewParser(input, false)Both adapters are in-memory. Choose strings.NewReader when you are starting with text and bytes.NewBufferString when surrounding code already works with byte buffers.
Set the inline flag correctly
The second argument to the documented constructor distinguishes context:
false: a complete stylesheet containing rules such asbody { ... }.true: declarations intended for an inline style attribute, such ascolor: red;.
Passing the wrong mode can produce unexpected grammar interpretation, especially when parsing declaration text rather than selectors and rules.
Parse a CSS string directly with Douceur
If you want a stylesheet object instead of manually iterating parser grammar, Douceur’s documented API accepts the string itself:
package main
import (
"fmt"
"log"
"github.com/aymerick/douceur/parser"
)
func main() {
cssText := `body { color: rebeccapurple; }`
stylesheet, err := parser.Parse(cssText)
if err != nil {
log.Fatal(err)
}
fmt.Println(stylesheet.String())
}Here there is no reader adapter because parser.Parse is designed to receive a string. Handle its returned error before using the stylesheet.
Parsing is different from loading CSS into a browser
In this context, “load” can mean several different operations:
| Operation | What it does | What it does not establish |
|---|---|---|
| Parse a string | Lexes text into tokens, grammar units, or a stylesheet object. | It does not render a DOM or apply computed styles. |
| Inline CSS in HTML | Rewrites CSS defined in an HTML document into inline style attributes. |
Douceur’s documented inliner does not fetch external stylesheets. |
| Browser rendering | Evaluates HTML, CSS, scripts, resources and layout in a browser engine. | A parser alone is not a browser. |
If your HTML contains <link rel="stylesheet" href="theme.css">, reading a CSS string into a parser will not retrieve that URL. Fetch the resource yourself, apply your own security policy, and then pass the fetched bytes or text to the parser. For email generation, use an inliner only when the CSS is available in the HTML document and the library’s supported syntax meets your needs.
Reusable helper functions
Reader-based helper
func parseCSS(cssText string) error {
input := parse.NewInput(strings.NewReader(cssText))
p := css.NewParser(input, false)
for {
grammar, _, data := p.Next()
if grammar == css.ErrorGrammar {
break
}
_ = data // transform or collect data here
}
if err := p.Err(); err != nil && err != io.EOF {
return err
}
return nil
}Return errors to the caller instead of panicking in a server, worker, or command-line tool that must continue handling other inputs.
Direct-string helper
func stylesheetFromString(cssText string) (string, error) {
stylesheet, err := parser.Parse(cssText)
if err != nil {
return "", err
}
return stylesheet.String(), nil
}Keep the original input available if you need to report a line, selector, or source fragment in an application error message.
Common mistakes and fixes
Passing a string where an io.Reader is required
A Go string does not implement Read. Wrap it with strings.NewReader or bytes.NewBufferString before constructing the parser.
Using the inline mode for a full stylesheet
Set the tdewolff parser’s isInline argument to false for rules and at-rules. Reserve true for declaration text from an inline style attribute.
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Ignoring parser termination and errors
The documented tdewolff loop stops at ErrorGrammar. Afterward, call Err() and distinguish normal end-of-input such as io.EOF from malformed input or another failure.
Expecting external imports or linked files to be fetched
Neither a reader adapter nor a parser automatically turns a URL into CSS. Resolve external resources explicitly, and do not assume Douceur’s inliner fetches them; its documentation says it parses CSS defined in the HTML document only.
Confusing parsing with sanitizing
A parser can recognize CSS syntax, but your application still needs a policy for untrusted input, resource URLs, custom properties, scripts in surrounding HTML, and any transformations you perform.
Assuming every CSS feature is supported
Verify the selected dependency’s version, supported CSS syntax, maintenance status and compatibility with your Go version. The documented examples establish the input APIs, not a complete compatibility matrix or performance benchmark.
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Performance, memory and reliability considerations
- String adapters avoid temporary-file I/O, but the CSS text remains resident in memory while the parser consumes it.
- For one-shot parsing, create a fresh reader for each input. A reader’s position advances and cannot be reused as if it were new.
- For large or streamed stylesheets, use an
io.Readerthat reads from the stream rather than first constructing one giant string, if the library supports that workflow. - Bound input size and execution time when CSS comes from users or remote services. Parsing errors should be observable and returned to the caller.
- Do not claim a speed advantage for either adapter without measuring your own workload; the available documentation does not provide comparable benchmarks.
Or skip the browser setup
If your real goal is to obtain a rendered screenshot of a page whose CSS is already loaded, you do not need to build a browser-capture pipeline. ScreenshotNeo is a website screenshot API and MCP server. It accepts cookie and consent banners before capture and removes more than 60 known consent platforms, newsletter popups and chat widgets; each step can be disabled. Bot checks, CAPTCHAs, blank pages, timeouts, failed loads and cache hits are not billed, and response headers identify the page verdict and billing result.
A single request returns PNG, JPEG, WebP or PDF. The API supports full-page captures with lazy images loaded, CSS-selector element capture, dark mode, device presets or custom viewports, retina scale, custom CSS and JavaScript, click-before-capture actions, selector hiding, selector/delay/network-idle waits, request and resource blocking, headers, cookies, user agents, authorization, timezone, geolocation, transparent backgrounds, resizing, configurable-TTL caching, signed image links, asynchronous webhooks, bulk capture of up to 100 URLs per call, usage reporting and an OpenAPI specification.
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpFor request parameters and output details, see the ScreenshotNeo documentation.
import requests
r = requests.get("https://api.screenshotneo.com/v1/shot", params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"}, timeout=90)
open("shot.webp", "wb").write(r.content)const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
const res = await fetch(`https://api.screenshotneo.com/v1/shot?${q}`);An MCP server provides take_screenshot, get_page_info and capture_pdf tools for Claude, Cursor and other MCP clients. The Free plan includes 1,000 screenshots per month without a card; paid plans start at $5 for 3,000 shots, and every feature is available on every plan. Sign up free for ScreenshotNeo.
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Quick decision guide
- Need parser tokens or grammar callbacks? Use tdewolff/parse with
strings.NewReaderorbytes.NewBufferString. - Need a parsed stylesheet from text with a compact call? Use Douceur’s
parser.Parse(cssText). - Need to inline CSS already present in HTML? Use the inliner workflow, while remembering it does not fetch external stylesheets.
- Need a rendered page image or PDF rather than a syntax tree? Use a browser-capable service such as ScreenshotNeo.
Frequently Asked Questions
Do I need to write the CSS string to a file first?
No. A reader adapter exposes the in-memory string directly to a reader-based parser.
Which adapter should I use, strings.NewReader or bytes.NewBufferString?
Both provide in-memory reader input. Use the one that matches the surrounding types in your code.
Can a CSS parser apply styles to HTML?
No. Parsing creates tokens or a stylesheet representation; browser rendering or an HTML inliner is a separate operation.
Will Douceur fetch a stylesheet referenced by a link tag?
Its documented inliner handles CSS defined in the HTML document and does not fetch external stylesheets.
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